The Lewisian geology of Gairloch, NW Scotland

The Lewisian Geology of Gairloch, NW Scotland

Geological Society Memoirs Society Book Editors A. J. FLEET (CHIEF EDITOR) P. DOYLE F. J. GREGORY J. S. GRIFFITHS A. J. HARTLEY R. E. HOLDSWORTH

A. C. MORTON N. S. ROBINS M. S. STOKER J. P. TURNER

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GEOLOGICAL SOCIETY MEMOIR No. 26

The Lewisian Geology of Gairloch, NW Scotland R. G. PARK

2002 Published by The Geological Society London

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Contents

Preface

vii

Acknowledgements

viii

Chapter 1: Introduction

1

1.1 Background 1.1.1 Wider significance of the Gairloch area 1.1.2 Description of the study area 1.1.3 Mapping history 1.2 Summary of the Lewisian geology of the NW Scottish mainland .2.1 Historical background to Lewisian chronology .2.2 Subdivision of the mainland Lewisian outcrop .2.3 The Scourian gneisses Origin of the gneisses .2.4 Scourian deformation and metamorphism .2.5 The Inverian event 1.2.6 The Scourie dyke swarm 1.2.7 The Loch Maree Group 1.2.8 Laxfordian modifications and younger events The early Laxfordian The late Laxfordian 1.3 Southern Region nomenclature

1 1 3 3 3 3 3 4 5 6 6 7 7 7 7 8 8

Chapter 2: The Early Stages - Scourian and Inverian

11

2.1 Petrography and distribution of the Archaean gneisses 2.1.1 Quartzo-feldspathic biotite gneisses 2.1.2 Hornblende gneisses 2.1.3 Amphibolites of the "early basic" suite 2.2 Chemistry and origin 2.2.1 Quartzo-feldspathic gneisses 2.2.2 Amphibolites 2.3 Structural chronology 2.4 Scourian and Inverian structure 2.4.1 Braigh Horrisdale 2.4.2 Creag Mhor Thollaidh 2.4.3 The lalltaig block 2.5 Metamorphism

11 11 12 12 13 13 14 14 16 16 17 18 18

Chapter 3: The Scourie Dyke Suite

21

3.1 Distribution and field relations 3.1.1 SW of the LMG belt 3.1.2 NE of the LMG belt 3.2 Petrography 3.3 Chemistry and origin

21 21 22 23 25

Chapter 4: The Loch Maree Group

29

4.1 Semipelites 4.1.1 Distribution 4.1.2 Petrography 4.1.3 Geochemistry and origin 4.1.4 Zircon age data 4.2 Other metasediment types 4.3 Carbonate (marble) 4.3.1 Distribution and petrography 4.3.2 Geochemistry 4.4 Banded-iron-formation

29 29 31 32 32 33 33 34 34

4.4.1 Distribution and petrography 4.4.2 Geochemistry 4.4.3 Origin 4.5 Graphitic pelite 4.6 Quartz-chlorite schist 4.6.1 Distribution and petrography 4.6.2 Geochemistry and origin 4.7 Garnet-biotite schist 4.7.1 Distribution and petrography 4.7.2 Geochemistry and origin 4.8 Origin of the BIF-carbonate-graphitic pelite assemblage 4.9 Amphibolites 4.9.1 Distribution and petrography 4.9.2 Geochemistry 4.9.3 Origin 4.10 Stratiform sulphide deposits within the Loch Maree Group 4.1 1 Origin of the Loch Maree Group 4.11.1 Semipelites 4.1 1.2 Other metasediments 4.1 1.3 Amphibolites

34 34 35 35 35 35 36 36 36 36 38 38 38 40 41

Chapter 5: Laxfordian Intrusive Rocks

45

5.1 The Ard-type gneisses 5.1.1 Distribution and structural relationships 5.1.2 Petrography 5.1.3 Geochemistry 5.1.4 Geochronology 5.2 Granite sheets 5.3 Granitoid pegmatites 5.3.1 Distribution and petrography 5.3.2 Geochemistry 5.3.3 Geochronology

45 45 45 47 47 47 47 47 48 48

Chapter 6: Laxfordian Structure and Metamorphism

51

6.1 Introduction 6.1.1 Structural overview 6.1.2 Deformation phases and chronology 6.1.3 Metamorphic conditions 6.2 Early Laxfordian (D1/D2) 6.2.1 Narrow marginal shear zones in Scourie dykes 6.2.2 SW of the Loch Maree Group outcrop 6.2.3 Supracrustal rocks of the Gairloch schist belt 6.2.4 The NE limb of the Tollie antiform 6.2.5 The western margin of the Creag Mhor Thollaidh block 6.2.6 The Ard gneiss 6.2.7 Relationship between D1 and D2 6.2.8 Change in plunge of L2 in the Ard gneiss outcrop 6.3 Large-scale D2 structure 6.4 Late Laxfordian 6.4.1 D3 minor structures 6.4.2 D3 major structures The Tollie antiform and Gairloch shear zone The Mill na Claise fold 6.4.3 D4 structures 6.5 Brittle-ductile shear zones 6.5.1 Early (D2) mylonitic rocks 6.5.2 D3 mylonites associated with the Gairloch shear zone

51 51 51 51 51 51 52 52 54

42 44 44 44 44

55 56 56 56 56 57 57 58 58 58 58 58 60 60

6.5.3 Later movements in the zones 6.6 Summary of Laxfordian tectonic history 6.7 Faults 6.7.1 Pre-Torridonian faults The Flowerdale fault N-S thrusts. N-trending strike-slip faults NE-trending normal faults 6.7.2 Post-Torridonian faults The Loch Maree fault NE-trending normal faults

60 61 62 65 65 65 65 65 65 65 65

Chapter 7: Tectonic Interpretation and Regional Significance

67

7.1 7.2 7.3 7.4

The Archaean (Scourian) 67 The Inverian: earlier Palaeoproterozoic? 67 The Scourie dyke swarm: events from c. 2.4 to c. 2.0 Ga? 67 The Laxfordian: later Palaeoproterozoic 68 7.4.1 The early Laxfordian: subduction-accretioncollision model 68 7.4.2 Later Laxfordian structural history 70 7.4.3 Post-D3 tectonic history 70 7.5 Palaeoproterozoic belts of the North Atlantic region 70 References

73

Preface

For more than five decades, the Lewisian complex of NW Scotland has been a training ground for British geologists, particularly structural and metamorphic geologists, and a testbed for new models and methods of investigation of the deep crust. Successive generations of geochronological techniques were first used in deciphering the complex history of this region. Yet, despite its popularity with British geologists, and its world-wide reputation as the most intensively studied piece of Precambrian crust, there are no modern Geological Survey sheet memoirs for any part of the mainland Lewisian; the famous, and impressively detailed, NW Highlands memoir of 1907 still stands after nearly a century as the best descriptive account available covering all aspects of Lewisian geology of the mainland outcrops. In contrast, the Outer Hebrides Lewisian is well served by an excellent modern memoir, although the accompanying map, at a scale of 1 : 100000, is insufficiently detailed to give a clear picture of the complexity of the more structurally interesting parts of that region. This situation contrasts with the vast amount of published information about specific aspects of Lewisian geology - structural, metamorphic, igneous and geochronological studies, tectonic overviews, and so on. The combination of detailed, large-scale, geological map and accompanying descriptive memoir is a valuable basis for subsequent specialized studies, and the present memoir

is an attempt to redress a deficiency by providing a large-scale coloured map (1 :20000) of one of the most critical and interesting parts of the Lewisian outcrop, sufficiently detailed to allow the relationships between the various rock types and the complex structures to be clearly displayed, together with a descriptive account covering the different aspects of the Lewisian geology. The Gairloch area, together with neighbouring Loch Maree, is the only part of the mainland Lewisian outcrop where Palaeoproterozoic supracrustal rocks and their relationships to the Archaean basement can be studied. Partly for this reason, it has been the subject of a considerable amount of research, spanning a period of more than forty years; it is readily accessible and frequently visited. The Lewisian crust represents a relatively small detached fragment of the Laurentian continent, yet it occupies a key position in the reconstructed jigsaw of a supercontinent made up of Laurentia, Siberia and Baltica, which existed at the end of the early Proterozoic period. The significance of Gairloch in this reconstruction lies in the Palaeoproterozoic rocks of the Loch Maree Group, representing an amalgamation of oceanic, trench, and arc assemblages with continental basement, which help to integrate Scotland into a Palaeoproterozoic collisional orogen stretching from the Torngat belt of Labrador through South Greenland and linking up with the Lapland-Kola belt of Scandinavia.

Acknowledgements

My research in the Gairloch area has spanned a period of over forty years, during this time many friends, colleagues and research students have helped me in a number of different ways. I am particularly indebted to Don Bowes at Glasgow University, who initially propelled me into the Lewisian and sustained me in the early years of my PhD work by his boundless enthusiasm, and John Tarney whose friendship and influence have been very important to me over many years, but especially, in prompting my rethink of

the tectonic interpretation of the Loch Maree Group. The memoir itself has benefited from rigorous and helpful reviews by John Mendum. Clark Friend and Rob Strachan. and from the careful attention of the Book Editor. Bob Holdsworth. Any remaining deficiencies are solely my responsibility. Finally I would like to thank my wife who has accompanied me during much of my fieldwork in the last few years and who has borne my neglect during the writing up with equanimity. R.G. Park. March. 2002.

Chapter 1 Introduction

1.1 Background

7.7.7 Wider significance of the Gairloch area

The Lewisian outcrops at Gairloch and Loch Maree have long been regarded as key areas in the Lewisian complex of NW Scotland because of the presence of the supracrustal rocks of the Palaeoproterozoic Loch Maree Group enclosed within the Archaean gneisses. Long before the age of this Group was known, attention was focused on these areas because the existence of metasedimentary rocks offered the possibility of establishing a stratigraphy within the Lewisian complex. Until relatively recently, Lewisian 'stratigraphy" was based entirely on the relative ages of igneous and metamorphic events. The early Geological Survey workers debated whether the acid gneisses represented deformed granites which intruded the metasediments or whether they represented an older basement on which the sediments had been laid down. During the discussion that followed the reading of the classic paper on the Lewisian by Sutton & Watson (1951), C. E. Tilley proposed that the critical region for interpreting Lewisian stratigraphy was neither Loch Laxford nor Loch Torridon, but Loch Maree and Gairloch. The pioneering work by Sutton & Watson on the Lewisian complex of NW Scotland stimulated the study of Precambrian basement complexes worldwide. Their methods offered the expectation that dyke swarms and other igneous suites could be used as stratigraphic markers to construct a chronology of tectonothermal events in what seemed hopelessly confusing gneiss terrains.

The Lewisian complex of NW Scotland is part of a widespread system of Palaeoproterozoic orogenic belts in the North Atlantic region. In a recent palaeomagnetic reconstruction of Laurentia and Baltica during the late Palaeoproterozoic to early Mesoproterozoic (Buchan et al. 2000; Fig. 1.1) the Nagssugtoqidian belt of East and West Greenland appears to continue through the Lewisian complex of NW Scotland to link up with the Lapland-Kola belt of Baltica. Comparisons of the tectonic histories of the Nagssugtoqidian and Lapland-Kola belts have been made by Bridgwater el al. (1990), Kalsbeek et al. (1993) and Park (1994, 1995)." There is a general consensus that the Nagssugtoqidian and Lapland-Kola belts, together with the Torngat and New Quebec orogens of Laurentia, are broadly coeval and have resulted from the progressive accretion of a number of terranes during the period 1.90-1.83 Ga (e.g. see references in Park 1995). While most of these orogenic belts show evidence of a continental margin setting, including subduction and marine sedimentation, and eventual continent-continent collision, such evidence had not been demonstrated for the Lewisian until recently. Previous views on the Palaeoproterozoic tectonic context of the Lewisian referred to an 'intraplate' setting (e.g. Park & Tarney 1987), reflecting the apparent lack of magmatic arc rocks and Palaeoproterozoic continental margin sediments, and the similarity of the Archaean basements on either side of the Palaeoproterozoic supracrustal belt.

Fig. 1.1. Reconstruction of Laurentia and Baltica during the Palaeoproterozoic. The Lewisian lies within a continuous Palaeoproterozoic belt extending from the Torngat belt of Laurentia through the Nagssugtoqidian of Greenland to the Lapland-Kola belt of Baltica. The arrows show the inferred movement directions. NAC, North Atlantic craton; nag. Nagssugtoqidian belt. From Buchan et al. (2000).

2

INTRODUCTION

Fig. 1.2. Simplified map of the Lewisian complex of NW Scotland, showing the main rock units and structures.

CHAPTER 1

The pivotal central position of the Lewisian in Laurentia-Baltica reconstructions warranted a re-evaluation of those lithotectonic units within the Lewisian that might imply a continent-continent collisional setting, in spite of contrary historical interpretations. This re-evaluation concentrated on the Loch Maree Group (LMG) and associated rocks of the Gairloch area (Park et al. 2001). The LMG, formerly thought to represent sediments and volcanics accumulated in an intra-continental extensional basin (Johnson et al. 1987), was re-interpreted as a marine sedimentary sequence laid down on oceanic plateau basalts (Park et al. 2001). Granitoid orthogneisses (including the Ard gneiss) interbanded with the LMG along its southwestern margin, were originally thought to be part of the Archaean basement, but have yielded a Palaeoproterozoic magmatic age. This is interpreted as evidence of a previously unknown magmatic arc within the Lewisian sector of the Nagssugtoqidian-Lapland-Kola collisional belt. This re-interpretation of the Lewisian geology of the Gairloch area is of critical significance in any reconstruction of the Palaeoproterozoic geology of the North Atlantic realm, and is partly the reason for this detailed review.

1.1.2 Description of the study area Gairloch lies within the southern region of the Lewisian outcrop of the NW Scottish mainland (Figs 1.2, 1.3). The area covered in this account (see Map) is about 75 km2 in extent, comprising the central part of the Gairloch inlier, from Loch Gairloch in the west to Loch Maree in the east, and bounded to the north by the GairlochPoolewe road. The southern boundary follows an irregular course from Loch Shieldaig south to Loch Braigh Horrisdale, then accompanies the southern margin of the inlier to Dubh Loch and Loch Bad an Sgalaig, from where it runs along the GairlochKinlochewe road to Slattadale on Loch Maree. Torridonian deposits overlie the Lewisian rocks on every side and also form several outliers within the main Lewisian outcrop. The Loch Maree Group occupies a central NW-trending belt flanked by acid gneisses of the Archaean basement complex. Topographically, the area is very rugged and well exposed, although not particularly high, the highest hills being around 400 m. The central part of the area is dominated by a series of steep, rocky ridges formed dominantly of amphibolite sheets, with the intervening metasedimentary schists forming smoother, less wellexposed slopes. More irregular, rocky and hilly terrain to the NE and SW is formed by acid gneisses cut by numerous amphibolite dykes. Much of the area is given over to sheep grazing. Although the terrain is rough, and in places difficult to negotiate, access is relatively easy; most parts of the area are within about 4 km from the A832, and there are several good tracks.

1.1.3 Mapping history The Gairloch area was first mapped in the late 1880s on a scale of six inches to one mile by C. T. Clough, B. N. Peach, J. Home, J. Gunn, and E. Greenly for the Geological Survey of Great Britain and described in the resulting NW Highlands memoir (Peach et al. 1907). The bulk of the mapping of the complex central part of the Gairloch area was carried out by C. T. Clough. Although impressively detailed, the early mapping was hampered by inadequate topographic base maps. Detailed 1 : 10000 re-mapping using aerial photographs, and accompanied by structural and geochemical work, was undertaken by Park (1963, 1964) followed by several other Glasgow PhD students, C. C. Bhattacharjee (1963, 1968), T. S. Ghaly (1966) and D. Elliott (1964). This work formed part of a Lewisian structural mapping programme under the supervision of D. R. Bowes. Further mapping was carried out in the northeastern part of the area by Park (1970a). The present map is based partly on the original mapping carried out by the author in the 1950s and 1960s (extensively checked and updated) and partly on new 1 : 10000 mapping carried out between 1995 and 1999 in

3

those parts not covered in the early mapping, chiefly in the southwestern and northern parts of the area. 1.2 Summary of the Lewisian geology of the NW Scottish mainland

1.2.1 Historical background to Lewisian chronology The first comprehensive account of the mainland Lewisian rocks appeared in the 'Northwest Highlands Memoir' (Peach et al. 1907) and established the following simple chronological sequence: an older assemblage of igneous rocks, in the form of acid gneisses, referred to as the 'fundamental complex', was intruded by a younger assemblage consisting of various intrusions including basic to ultrabasic dykes of the 'Scourie dyke' suite. Both assemblages of rocks were subsequently affected by deformation, which caused severe modifications to the complex in certain areas, particularly the northern and southern parts of the mainland, but left the central part comparatively unscathed. The chronological subdivision of the Lewisian was re-interpreted by Sutton & Watson (1951) in terms of successive orogenic cycles: the older, corresponding approximately to the 'fundamental complex' of Peach et al. (1907), was termed the Scourian, and the younger, the Laxfordian. The two cycles were separated by the intrusion of the Scourie dykes, which Sutton and Watson regarded as anorogenic and all intruded at essentially the same time, geologically. Thus the concept arose of Scourian rocks, formed during Scourian time, reworked during Laxfordian time. This 'stratigraphical' interpretation was modified by the work of Tarney (1963), Park (1964), Evans & Tarney (1964), and Evans (1965) which showed that a third major tectono-metamorphic event, the Inverian', took place before, and possibly during, the period of Scourie dyke emplacement. The similarity of structural style and orientation, and of metamorphic facies, between the Inverian and the later Laxfordian led to considerable confusion, and structures could only be assigned confidently to one or the other event where Scourie dykes could be seen either to cut or be affected by the structures in question. Park (1970b) proposed the term 'Badcallian' for the granulite-facies metamorphic event within the Scourian period. Many authors have used the term 'Scourian' in a different sense to that of Sutton & Watson (1951), to include only Lewisian rocks and events of Archaean age, prior to the Inverian event. For these reasons, it is necessary to clarify the usage of the terms Badcallian and Scourian. The terrane model for the Lewisian proposed by Friend & Kinny (2001) implies the possibility of separate origins of the various Archaean blocks making up the Lewisian complex; using the term 'Scourian' for all of these may disguise separate provenances. In the absence of an agreed redefinition of the Scourian, the term is used here to cover all the Archaean components of the Lewisian complex. However the term 'Badcallian' is restricted to the granulite-facies metamorphic event of the type area in the Central Region.

7.2.2 Subdivision of the mainland Lewisian outcrop The Lewisian complex of the mainland was divided by Peach et al. (1907) into three separate regions - northern, central and southern (Fig. 1.2). The Central Region extends from north of Scourie to north of Gruinard Bay, and is composed typically of granulitefacies Scourian rocks relatively unmodified by younger Laxfordian effects. These rocks are intruded by the generally undeformed but metamorphosed, sub-vertical, mafic and ultramafic dykes of the Scourie dyke swarm with a northwesterly to westerly trend. The Northern and Southern Regions, on the other hand, represent belts where the original Scourian gneisses have been strongly modified by Laxfordian deformation and amphibolitefacies metamorphism (Fig. 1.2). The only Palaeoproterozoic

4

INTRODUCTION

Fig. 1.3. Simplified map of the Lewisian outcrop of the Southern Mainland region, showing the location of the Loch Maree Group and the main structural elements. The box shows the area mapped and described in this memoir. The blank areas are occupied by Torridonian rocks. The boundary between the Southern and Central regions lies NE of Little Loch Broom, in the NE corner of the map.

metasedimentary units within these two regions occur in the Loch Maree Group, at Loch Maree and Gairloch, in the Southern Region (Fig. 1.3), although other units occur on South Harris in the Outer Hebrides. The Southern Region (Figs 1.2, 1.3) extends from Gruinard Bay to Loch Torridon, and includes the islands of Rona and Raasay. In the northeastern part of the region, extending from Gruinard Bay to Fionn Loch, north of Loch Maree. Inverian deformation and amphibolite-facies metamorphism have severely modified the Scourian gneisses. The Scourie dykes are little affected here by

Laxfordian deformation, which only becomes intense around Fionn Loch and Loch Maree. and further south.

1.2.3 The Scourian gneisses Like the earliest elements of many other Archaean high-grade terrains, the Scourian rocks of the Central Region are predominantly banded, and less commonly massive, grey gneisses of tonalitic. granodioritic. or trondhjemitic composition, with minor sheets or lenses of sranitic gneiss, and numerous mafic and

CHAPTER 1

5

Table 1.1. Simplified Lewisian chronology

ROCK-FORMING PROCESS

Ga

TECTONOTHERMAL EVENT

-3.0

components of Archaean complex (1,2)

— 2.8 'Scourian'at Gruinard (5)

Badcailian at Scourie (1,3,4)*

2.6

Inverian (4) first Scourie dykes (6) -

2.4

2.2-

Loch Maree Group (7); 2nd Scourie dykes (6)

2.0

Ard gneiss (8)

Laxfordian D 1 / D 2 ( 8 )

— 1.8 Laxfordian D3 (8,9)

Toilie pegmatites (8) -1.6-

- 1.4

-

Laxfordian D4 (9)

1.2'Grenvillian' (9)

- 1.0-

References: 1, Friend & Kinny 1995; 2, Whitehouse 1989; 3, Humphries & Cliff 1982; 4, Corfu et al. 1994; 5, Corfu et al. 1998; 6, Heaman & Tarney 1989; 7. Whitehouse et al. 1997a; 8, Park et al. 2001; 9, Moorbath & Park 1971. * The age of the type Badcallian granulite-facies event has not been conclusively determined; dates of c. 2.7 and 2.49 Ga have both been suggested (see text).

ultramafic layers and enclaves. These grey gneisses make up some 75 to 80% of the complex in the unmodified parts of the Central Region. Quartz-plagioclase-hornblende-(pyroxene) gneisses in which hornblende aggregates have replaced pyroxene are the most abundant type. Friend & Kinny (1995) infer an age of c. 2960 Ma from single zircon studies for the protoliths of the tonalitic gneisses from the Scourie area. The more mafic enclaves within the acid gneisses, previously collectively termed the 'early basic' bodies, vary in size from a few centimetres to about a kilometre across, and are particularly common in the Scourie and Assynt areas. In the Central Region, such bodies typically contain both clinopyroxene and orthopyroxene, and variable amounts of hornblende, in addition to plagioclase. They are typically cut or veined by acid gneiss, locally becoming more numerous and grading into agmatite. In places, patches of acid gneiss enriched in small mafic clots may represent a further stage in the disruption of these mafic enclaves. The ultramafic enclaves, which are normally associated with mafic material, vary from monomineralic masses of hornblende or pyroxene, to large bodies of mainly peridote or dunite, either homogeneous or banded in nature, and with varying proportions of hornblende (Bowes et al. 1964). Ultramafic/mafic complexes near Scourie also contain anorthosite layers and are associated spatially with pelitic metasediments (Davies 1974). The mafic/ultramafic bodies appear to be older than the acid gneisses and to have been invaded by them. The mafic/ultramafic bodies at Achiltibuie, Drumbeg, and Scouriemore have yielded Sm-Nd whole-rock ages

of , , and 0 Ma respectively, interpreted as emplacement ages (Whitehouse 1989). A few narrow bands of metasediment are associated with the mafic/ultramafic units (Okeke et al. 1983; Cartwright et al. 1985). These consist mainly of rusty-weathering, biotite-muscovite semipelites, and are cut by mafic gneisses, although their relationship to the tonalite gneisses is unclear. Quartz-perthite pegmatites are widely distributed. These bodies cut the gneissose banding, and are associated with local retrogression of the granulite-facies assemblage to amphibolite facies. They have yielded U-Pb zircon ages of 2490-2480 Ma (Corfu et al. 1994) and have been taken as marking the boundary between the Scourian and Inverian events (Evans & Lambert 1974; Table 1.1). It is difficult to determine the original nature of the gneisses in the Northern and Southern Regions because of the effects of Laxfordian reworking. Biotite- and muscovite-bearing quartzofeldspathic gneisses predominate, although hornblende-bearing varieties also occur. Mafic and ultramafic enclaves, similar to those of the Central Region are common. The gneisses in both Northern and Southern Regions are generally in amphibolite facies although relict granulite-facies assemblages are found locally in the northern part of the Southern Region. Origin of the gneisses. Sm-Nd and U-Pb zircon data indicate that almost the whole crust of the mainland Lewisian originated in the interval 3000-2700 Ma (Whitehouse 1989; Whitehouse et al. 1997b; Corfu et al. 1998). The bulk of the gneisses are thought to be of

6

INTRODUCTION

Fig. 1.4. Cartoon illustrating a possible method of generation of Scourian crust, from the progressive underplating of mafic oceanic material overlain by marine sediments, accompanied by the emplacement of tonalitic magmas generated by melting of the underplated material (from Park & Tarney 1987).

plutonic igneous origin as originally suggested by Peach et al. (1907). Geochemical studies of the gneisses of the Central Region (Weaver & Tarney 1980) indicate that they have a bimodal character. The mafic components show a range of Fe/Mg ratios and their trace-element patterns are consistent with fractionation of low-pressure, tholeiitic basaltic magmas. The common association of ultramafic/mafic bodies with metasedimentary layers suggests that this material represents fragments of ocean-floor crust intercalated teconically within the continental crust. The tonalitic to trondhjemitic gneisses on the other hand have rare-earth-element patterns consistent with partial melting of a mafic source under high-pressure hydrous conditions. Tarney & Weaver (1987a) envisage a process of relatively shallow melting at a low-angle subduction zone where melts generated under hydrous conditions would yield relatively dense tonalitic magmas which solidify at deep levels and progressively thicken the crust by underplating. Thus the mafic/metasediment (oceanic crust) association would first experience a high-grade metamorphic phase at the base of the continental crust before being uplifted by further underplating (Fig. 1.4). The ubiquitous tectonic disruption of the deeper parts of the complex may be attributed to long periods of ductile-shear deformation affecting the base of the accreting crust as underplating proceeded. This model explains the concentration of mafic and sedimentary material which originated at shallow crustal levels, in the deeper parts of the complex. The granulite-facies terrain of Scourie and Assynt in the Central Region is characterized by a high proportion of intercalated ultramafic and mafic material, and the composition of the grey gneisses varies from mafic diorite to tonalite with only a small proportion of silicic trondhjemites (Sheraton et al. 1973). However the northern part of the Southern Region around Gruinard Bay consists predominantly of amphibolite-facies trondhjemitic gneisses with numerous mafic enclaves (Rollinson & Fowler 1987). The gneisses elsewhere in the Southern Region, and in the Northern Region, display lower proportions of mafic material, and few ultramafic enclaves; they are more silicic and potassic, with a significant proportion of granodioritic material. These petrological and geochemical variations may reflect differences in crustal history, as well as in original crustal level, across the Lewisian outcrop. Whereas the gneisses of Scourie experienced granulitefacies metamorphism at up to 11 kb pressure, corresponding to depths of at least 35km (see below), there is no evidence that the gneisses to the north and south were metamorphosed at such high pressures and the presently exposed rocks represent originally higher crustal levels. Friend & Kinny (2001) point to significant differences in protolith ages between Northern (2770-2840 Ma), Central (2960-3030 Ma) and Southern (c. 2840 Ma) regions, and the lack of evidence for the c. 2.49 Ga metamorphic event in either the Northern or Southern Regions, in support of their model of terrane accretion in the Proterozoic.

1.2.4 Scourian deformation and metamorphism The extreme heterogeneity of the complex, coupled with the almost

ubiquitous compositional banding, are typical of plutonic igneous rocks affected by major shear zones in basement complexes, and most of the protolith rocks appear to have been subjected to intense deformation. The banding or foliation is typically sub-horizontal or gently inclined over large areas of the Central Region (see Sheraton et al. 1973). although locally steepened in late Scourian shear zones. In the Southern Region, the foliation is characteristically steeply dipping and has a northerly or northeasterly trend where unmodified by subsequent deformation. Granulite-facies metamorphism attributed to the Badcallian event is a characteristic feature of the Scourian complex of the Central Region, although retrogression to amphibolite facies is widespread (see Sills & Rollinson 1987). Geothermometric and geobarometric data for the Badcallian event indicate peak temperatures of 1000°C and pressures of 10kb. with a subsequent steady decrease in both temperature and pressure (Barnicoat 1987). The granulite-facies Badcallian metamorphism has proved difficult to date. A metamorphic event in the Scourie type area of the Central Region dated at about 2500 Ma was identified by Corfu et al. (1994) and Friend & Kinny (1995). This event may correspond to either the Badcallian or to Inverian recrystallization of an earlier (c. 2.7 Ga?) Badcallian metamorphism (cf. Friend & Kinny 2001). A study of U-Pb systematics at Gruinard Bay in the Southern Region (the nearest dated Archaean outcrop to Gairloch) provides convincing evidence for the existence of a significant intrusive metamorphic event there at 2730 Ma but no trace of an event around c. 2.5 Ga. suggesting that the latter event was confined to the Central Region (Whitehouse et al. 1997b: Corfu et al. 1998).

7.2.5 The Inverian event The type area for the Inverian event is the l-2km-wide. WNWtrending, Canisp shear zone (Tarney 1963; Evans 1965; Attfield 1987) which cuts through the middle of the Central Region in the Assynt district (Fig. 1.2). A second major shear zone of Inverian age occurs at the northern margin of the Central Region. This zone is approximately 4km wide, trends NW. and extends from near Scourie to Loch Laxford (Fig. 1.2: Beach et al. 1974: Davies 1978). On its SW side, it cuts Scourian structures and causes retrogression of the granulite-facies gneisses to amphibolite facies. On its NE side, it becomes intensely affected by Laxfordian deformation in the Laxford shear zone (see below). The Inverian event has proved difficult to date because of overprinting by subsequent Laxfordian metamorphism. The event postdates (but is assumed to be closelyrelated to) a late Badcallian pegmatite suite in the Assynt type area and is dated by Corfu et al. (1994) at 2490-2480 Ma. An important shear zone, structurally similar to the Inverian zones of the Central Region, occurs at the NE margin of the Southern Region between the Gruinard River and Fionn Loch, with a width of about 8 km (Fig. 1.3). This zone is overprinted and obscured by the major Laxfordian belt of the Southern Region on its SW side (Crane 1978; Park et al. 1987). Corfu et al. (1998) were unable to detect an event of Inverian age in their detailed geochronoloeical studv of the Gruinard Bav area

CHAPTER 1

where the Inverian-type event has resulted in intense deformation and metamorphism but the rocks are comparatively little affected by the Laxfordian overprint. The correlation between the type Inverian of the Central Region and the pre-dyke structures and metamorphism referred to the Inverian in the Southern Region must therefore be regarded as unproven. 1.2.6 The Scourie dyke swarm The Scourie dykes are typically steep, with a northwesterly to westerly trend, and for the most part appear to have been emplaced as a result of dilation, implying considerable crustal extension. They are thickest and most numerous in the Southern Region between Gruinard Bay and Torridon. In many areas, especially between Gairloch and Loch Torridon, they are significantly controlled by the pre-existing structure, becoming thinner and more numerous in zones of strong Inverian foliation (Park & Cresswell 1972, 1973). Tarney & Weaver (1987b) define four petrological/geochemical types: bronzite-picrites, norites, olivine-gabbros, and quartz-dolerites. The quartz-dolerites are by far the most abundant. Evidence from igneous assemblages in dykes from the Central Region indicates emplacement at depth into hot country rock (O'Hara 1961; Tarney 1963). The timing and duration of Scourie dyke intrusion is still uncertain. Rb-Sr whole-rock ages of about 2.40 Ga (Chapman 1979) and Sm-Nd mineral ages of 2.0 Ga (Cohen et al. 1988) from quartz-dolerites have been complemented by precise U-Pb baddeleyite ages of 2418 + 7-4 Ma for a bronzite picrite and 1992 + 3-2 Ma for an olivine-gabbro (Heaman & Tarney 1989). These data are all from the Central Region. Thus the main Scourie dyke swarm in the Central Region was emplaced around 2400 Ma ago, at crustal depths of 10-20 km, during the Inverian metamorphism (see Dickinson & Watson 1976), but certain members of the swarm were emplaced much later (c. 2000 Ma) into cooler crust. However no emplacement ages have been published from the dykes in the Northern or Southern regions and in view of the possibility that the three regions may not have coalesced until after the dyke emplacement, the equivalence of the swarms in the three regions has not been proved. There is no evidence of deformed Scourie dykes cut by undeformed younger members of the suite, but it is possible that some tectonic activity took place between the earlier and later dyke emplacement episodes, potentially giving rise to confusion in nomenclature between the Inverian and Laxfordian events (see Section 1.2.8). 1.2.1 The Loch Maree Group Two belts of metasediment, with a combined outcrop area of about 130 km2, were first described by Peach et al. (1907) at Loch Maree and Gairloch in the Southern Region. Both outcrops exhibit intense polyphase deformation. Peach et al. were unable to decide whether the sediments were older than the igneous rocks of the fundamental complex, or were unconformable upon them (see discussion in Peach & Home 1930). They noted that the boundaries were tectonically modified and that gneisses appeared to be thrust over metasediments at Loch Maree. Park (1964, 1965) concluded from a comparison of their structural and metamorphic histories, that the sediments were younger, and this view was confirmed by O'Nions et al. (1983) who obtained Sm-Nd model ages of 2490 and 2190 Ma on two samples of clastic metasediment, indicating an important post-Archaean component in their provenance. More recent U-Pb dating of zircons in semipelitic metasediments (Whitehouse et al. 1997a) indicates a range of ages for the source material, with a significant component at c. 2000 Ma, which is taken as the probable depositional age (Table 1.1). Tonalite to granodiorite gneisses (the Ard gneiss) intruding into the supracrustal rocks have yielded a precise U-Pb zircon age of 1907 Ga (Park et al. 2001; Section

7

5.1.4).

The supracrustal assemblage consists of a thick sequence of amphibolites, of probable volcanic origin (Park 1966; Johnson et al. 1987; Park et al. 2001), intercalated with semipelite schist and minor layers of carbonate, banded-iron formation; and graphitic pelite. It is described in detail in Chapter 4. From the geochemistry of the supracrustal rocks, Park et al. (2001) suggest that the Loch Maree Group represents an accretionary complex in which slices of ocean-floor material have been juxtaposed tectonically with continental- or arc- derived clastic material at an active margin. In this interpretation, the outcrop of the Loch Maree Group represents a Palaeoproterozoic collisional suture. 1.2.8 Laxfordian modifications and younger events The problem of nomenclature for those tectono-metamorphic events overlapping the emplacement of the Scourie dykes has already been alluded to. In practice, structures and metamorphism affecting any of the 'Scourie' dykes have been regarded as Laxfordian, following Sutton & Watson (1951), despite the fact that the age of the dykes in the Northern and Southern regions is unproved. Laxfordian modifications on the mainland can be divided simply into an earlier set associated with amphibolite-facies metamorphism and the emplacement of granites and pegmatites, and a later set accompanied by retrogressive alteration to greenschist facies or lower (Table 1.1). The earlier Laxfordian deformations produced fabrics in the Scourie dykes associated generally with amphibolitefacies recrystallization of the original igneous assemblages. In many areas, recrystallization has occurred in the absence of deformation, producing the typical relict sub-ophitic dyke textures of the Central Region. This static recrystallization may represent a continuation of the Inverian metamorphic event. The early Laxfordian. This is the principal and most widespread phase of Laxfordian deformation; it post-dates the Loch Maree Group, and also affects the 1907 Ga Ard gneiss that intrudes these supracrustal rocks (Park et al. 2001). The Ard gneiss date thus gives an upper limit to the age of the early Laxfordian event. The end of the main (high-grade) Laxfordian metamorphism is indicated very approximately by a grouping of metamorphic ages, including RbSr whole-rock, lead-isotope, and zircon ages in the range 1860-1630 Ma, but concentrated at about 1700 Ma (Lambert & Holland 1972; Lyon et al. 1973) (Table 1.1). A lower age limit is given by the widespread Laxfordian granite/pegmatite suite, which is posttectonic to the early structures. A pegmatite from this suite has given a precise U-Pb zircon age of 1694 5 Ma (Park et al. 2001). The early Laxfordian structures are heterogeneous in their development. In the Central Region, they are mainly confined to narrow shear zones only a few metres in width, with the exception of the 1 km-wide Canisp shear zone (see Attfield 1987). The main belts of Laxfordian reworking are situated in the Northern and Southern regions (Fig. 1.1). In both regions, the first Laxfordian foliation is associated with variable, locally very intense, deformation, with pre-existing structures being rotated into parallelism with the new fabric. Near the margins of the Laxfordian belts in the Southern Region, Laxfordian structures are confined to steep planar fabrics within, and generally sub-concordant with, the Scourie dykes. These progress from narrow marginal zones to encompass the whole width of the dyke, then spread out into the host gneisses. Park et al. (1987) show that the fabric formed during the first phase of the Laxfordian deformation in the Southern Region is folded during a second phase to become flat-lying between Carnmore and Gairloch, and south of Loch Torridon (Fig. 1.2). Thorough reworking of the Scourian gneisses has produced a finely banded 'Laxfordianized' gneiss with concordant and locally lenticular amphibolite sheets (interpreted as deformed Scourie dykes), and in parts pervaded by granitic migmatite of Laxfordian

8

INTRODUCTION

Fig. 1.5. Cartoon diagrams showing possible Laxfordian D 2 and D 3 structure of the Lewisian (after Coward & Park 1987). Note that these are composite sections, incorporating both the mainland and Outer Hebrides. (A) D 2 structure represented as gently inclined shear zones with relative movements approximately perpendicular to the page. (B) D 3 structure, showing upright or moderately inclined folds and steep shear zones: movement direction oblique to page, with a component of NE-SW compression.

age. "Laxfordianized' gneisses of this kind are typical of those parts of the Southern Region south of Loch Torridon, and also of the Northern Region, north of Loch Laxford. Large bodies of granite are confined to the Loch Laxford area, where several thick sub-concordant sheets of pink, foliated granite occur within a zone 2-4 km wide. North of the granite sheets, narrow bands of granite and pegmatite are abundant. Coward & Park (1987) proposed that the main Laxfordian belts of the mainland were linked in a mid-crustal shear zone network that separated and enclosed more stable crustal blocks, whose relative movement gave rise to the observed structures. These authors considered that the Laxfordian D 1 and D 2 deformations recognized on the mainland probably represent earlier and later stages of a progressive deformation involving the transport of higher-level crustal blocks relative to lower, on a major, subhorizontal, shear zone (Fig. 1.5). This major shear zone is exposed in the Northern and Southern Regions of the mainland, but was believed to pass beneath the Central Region. It may be more widely represented in the Outer Hebrides, where a lower crustal level of the Laxfordian is exposed. The late Laxfordian. Later Laxfordian structures include the prominent NW-trending folds (Laxfordian D 3 ) that dominate the outcrop pattern of the Laxfordian belts (Fig. 1.2) such as the Tollie and Torridon antiforms in the Southern Region (Fig. 1.3). The Tollie antiform in the Gairloch area is associated with the formation of the major Gairloch shear zone (Odling 1984; Park et al. 1987) which is about 6km wide. These structures are associated with the development of a locally developed, planar fabric accompanied by retrogression to greenschist facies. The age of this event is deduced from a cluster of K-Ar hornblende dates of

around 1700 Ma (Moorbath & Park 1971) and an upper limit is given by the 1694 Ma zircon age on a deformed pegmatite folded by the Toflie antiform (Park et al. 2001: Table 1.1: Section 6.4). The regional late Laxfordian folds were superseded by more localized structures of various styles and orientations, together with brittle-ductile shear zones. These later structures were assigned by Park (1964) to the late phase' (Laxfordian D4) of the Laxfordian in the Gairloch district, and have been described in various parts of the Laxfordian belts by Bhattacharjee (1968). Dash (1969). and Cresswell (1972). This late Laxfordian deformation may have occurred around 1500 Ma. corresponding with a period of significant resetting of K-Ar systems in hornblendes (Moorbath & Park 1971: Table 1 . 1 ) . Two younger K-Ar dates of 1148 Ma and 1169 Ma were obtained by Moorbath & Park ( 1 9 7 1 ) from chloritized biotite in acid gneisses from Torridon. These ages are close to a biotite Rb-Sr age of 1160 Ma reported by Giletti et al. (1961). and suggest that some of the later structures in the Lewisian complex (e.g. certain crush belts) may result from possible Grenville-age movements at around 1100 Ma (Park 1970a). Evidence for activity of the same age north of the Langavat shear zone in South Harris, has been presented by Cliff & Rex (1989).

1.3 Southern Region nomenclature Some explanation is necessary for the nomenclature adopted in succeeding chapters of this memoir for the rocks and events of Southern Region, bearing in mind the problems discussed in previous section. The terms Scourian. Inveriau. Scourie dyke and Laxfordian used for convenience for the respective events and rocks in

the the the are the

CHAPTER 1

Southern Region based on previous practice. Thus the term 'Scourian' is retained for all Archean gneisses in the absence of an agreed alternative, although they are probably of different age to the type Scourian gneisses of the Central Region. More critically, the lack of geochronological evidence for the equivalence of the dyke swarm to the dated type Scourie dykes of the Central Region, implies that there is no sound evidence for the equivalence of the Tnverian' events either. Thus, although the main Laxfordian

9

event can probably be correlated throughout the Lewisian outcrop, the preceding events cannot, and their nomenclature is based on analogy to the similar structures and rocks of the Central Region. If Friend & Kinny (2001) are correct in their belief that the various terranes making up the Lewisian complex did not come together until the Laxfordian, the correlation of the Inverian event and the Scourie dykes between the two regions becomes more difficult to justify.

Chapter 2 The Early Stages - Scourian and Inverian Archaean gneisses occupy large areas on each side of the central NW-SE-trending belt formed by the outcrop of the Loch Maree Group (Fig. 2.1). They are cut by numerous amphibolite dykes of the "Scourie dyke' swarm (see Chapter 3). The gneisses are predominantly granodioritic to tonalitic, quartzo-feldspathic biotite gneisses but large areas of more mafic hornblende gneiss occur in the NE, and small bodies of amphibolite are enclosed within the gneisses in all parts of the area. The gneisses have undergone a long and complex history, having experienced Scourian, Inverian, and Laxfordian thermo-tectonic events. Although Scourian structures have been preserved locally, little of the original Scourian mineral assemblage remains, and the

mineral assemblages mainly reflect Inverian and Laxfordian recrystallizations (see Section 2.4).

2.1 Petrography and distribution of the Archaean gneisses

2.1.1 Quartzo-feldspathic biotite gneisses The quartzo-feldspathic biotite gneisses are pale grey to pinkishweathering, banded or massive, granodioritic (or less commonly tonalitic) gneisses containing biotite as their main mafic mineral. The banded varieties show partial or complete segregation of micas

Fig. 2.1. Simplified geological map of the Gairloch area, showing main rock units and structures.

12

THE EARLY STAGES - SCOURIAN AND I N V E R I A N

Fig. 2.2. Coarsely banded granodioritic 'Scourian' gneisses, east of Loch Braigh Horrisdale. Note F2 folds to left of hammer. Banding is S1 transposed into S2.

into separate layers or lenses (Fig. 2.2). Typical examples contain oligoclase, quartz, microcline and a dark brown biotite, in varying proportions. Muscovite, chlorite or epidote may be present in addition, together with traces of opaque ore and apatite. The gneisses within several kilometres of the outcrop of the Loch Maree Group exhibit evidence of partial recrystallization from a coarsergrained assemblage, the larger feldspar grains being surrounded by granular aggregates of smaller grains of feldspar and quartz. Chlorite and epidote are clearly replacive. Retrogressive recrystallization to an epidote-bearing assemblage is particularly marked on the SW limb of the Tollie antiform, where the medium- to coarse-grained gneisses typical of the central and eastern parts of the Tollie area are transformed to finer-grained, striped, and closely-foliated varieties with a quartz-oligoclasemicrocline-biotite-epidote assemblage, carrying sphene and minor opaques as accessories. The mean grain size of the larger feldspars varies markedly from around 0.5 mm in the central Tollie area to 0.2-0.3 mm in the western Tollie belt, and down to around 0.1 mm in the westernmost highly sheared zone. A similar variation is evident SW of the Loch Maree Group outcrop from around 1 mm near Loch Braigh Horrisdale to around 0.1 mm immediately SW of Shieldaig Lodge (see map; Fig. 2.1). The banding in the gneisses of the central Tollie and Loch Braigh Horrisdale areas is on a centimetre to metre scale (Fig. 2.2) whereas near the central schist belt containing the Loch Maree Group, this structure is replaced by a fine striping on a millimetre scale. Typically, biotite does not show obvious crystallographic alignment except in the recrystallized striped gneisses where the micas exhibit a strong planar fabric. 2.7.2 Hornblende gneisses Quartzo-felspathic gneisses containing hornblende as their principal mafic constituent occur widely in the area NE of the outcrop of the Loch Maree Group. They are generally so interbanded or mixed with biotite gneisses that it has not proved possible to map their boundaries.

In the northeastern part of the district, east of the Leth-chreige crush belt, metadioritic to metagabbroic gneisses occupy a broad belt, around 1 km in width, extending from the summit of Creag Mhor Thollaidh in the NW to the peninsula of Rubh Aird an Anail in the SE. Typical examples contain hornblende and plagioclase with minor quartz and biotite. The more metagabbroic rocks contain large hornblende grains sieved with quartz, indicating retrogression from pyroxene. In samples from the southeastern part of this outcrop, partially retrogressed clinopyroxene and serpentinized olivine occur in addition. Both banded and massive varieties of these rocks occur, and in many places they are veined by, or mixed with, quartzo-feldspathic material. In the southwestern part of this belt, metadioritic gneiss and quartzo-feldspathic hornblende gneiss are interbanded, and it is impossible to define a clear boundary to the metadiorite outcrop. A second large outcrop of hornblende gneisses containing abundant mafic enclaves occurs on. and SE of, Meall an Doirean, and are folded around the hinge of the Tollie antiform (see map). Hornblende gneisses also form the dominant rock type on the peninsula of Ard lalltaig, where mafic and ultramafic rocks are intimately mixed and invaded by leucocratic material. 2.1.3 Amphibolies of the 'early basic' suite Numerous small bodies of metabasic amphibolite occur in the form of narrow concordant sheets and lenses throughout the outcrop of the acid gneiss complex (Fig. 2.3B). These bodies are generally too small to map, but a few larger bodies have been mapped out. One sheet occurs west of the crest of the Tollie antiform. SE of Loch Laraig [848 768], and several small masses of the order of tens of metres across occur in the area SW of Mullach nan Cadhaichean (in km squares [81 69] and [82 69]. These bodies may easily be distinguished from the adjoining Scourie dykes by their banded and veined appearance. In some cases they are agmatitic at their margins and may grade into agmatitic gneiss or into zones of gneiss containing abundant mafic pods and lenses. These rocks typically contain hornblende and

CHAPTER 2

13

Fig. 2.3. (a) Banded Archaean gneisses affected by intense Laxfordian D2 deformation, near crest of Tollie antiform, SE of Loch Tollie [845 776]. (b) Lensshaped ultramafic inclusion in granodioritic Buainichean gneiss, east of Am Feur-loch [865 725].

plagioclase, with minor quartz, epidote, or biotite, and titanite as accessory. The veins and stripes are dominantly feldspathic, with minor quartz. Ultramafic varieties, consisting mainly of hornblende and minor plagioclase, also occur, but bodies of mappable size are rare. One small lens occurs near the Gairloch-Kinlochewe road 1 km east of Am Feur-loch at [865 725] (Fig. 2.3b).

2.2 Chemistry and origin 2.2.7 Quartzo-feldspathic gneisses Twelve major-element analyses of the quartzo-feldspathic gneisses of the Gairloch district were carried out by the author (Park 1963), nine from the western Tollie gneisses, on the NE side of the central schist belt, and three from the Shieldaig gneisses on the SW side of the supracrustal belt (Table 2.1). Further analyses were carried out by Holland & Lambert (1973, 1995) as part of a general geochemical survey of the Lewisian gneisses of the Scottish mainland outcrop. Holland & Lambert provide average analyses of the gneisses around Sidhean Mor, (the Shieldaig gneisses), the gneisses of Creag Mhor Thollaidh, and those of the central and western zones of the Tollie antiform (Table 2.1, columns B-E). As would be expected of such heterogeneous rocks, the analyses of Park (1963) show considerable variation in major elements: e.g.

SiO2 56.5-72.7; A12O3 13.7-20.2; Fetot 1.1-4.8; MgO 0.6-2.3; CaO 1.0-4.3; Na2O 4.3-7.6; and K2O 0.4-4.8. The wide ranges shown reflect a correspondingly wide variation both in the proportion of mafic minerals (chiefly biotite) in the gneisses at hand specimen scale, and also in the K 2 O/Na 2 O ratio. They range in composition from granodioritic to tonalitic varieties. The average analyses of Holland and Lambert (1973, 1995) fall within or close to the above ranges with the exception of the Creag Mhor Thollaidh gneisses, which are significantly enriched in FeO, MgO, and CaO, and impoverished in K2O, compared with the other gneisses, reflecting their more 'dioritic' character. The average analyses of Holland and Lambert (1973) (Table 2.1) probably represent mixtures of several different protoliths, ranging from granodiorite through tonalite to diorite, whose boundaries are now obscured in the field, and should not be regarded as representing any one original protolith. Nevertheless they provide a useful indicator of the balance between the original protolith types in the different areas. On the basis of their regional survey, Holland & Lambert (1973) considered that the more mafic Tollie gneisses were chemically similar to the tonalitic gneisses of the Gruinard Bay district. The Gruinard bay gneisses are described by Rollinson & Fowler (1987) and a comparison of their average analyses of tonalitic gneiss with the Holland and Lambert samples (Table 2.1) shows that the Gruinard Bay samples are poorer in MgO and slightly richer in

THE EARLY STAGES - SCOURIAN AND I N V E R I A N

14

Table 2.1. Chemical analyses (major elements) of Scowian gneisses

SiO2 A12O3 FeTot

MgO CaO Na 2 O K2O

A

B

C

D

E

F

56.5-72.7 13.7-20.2 1.1-4.8 0.6-2.3 1.0-4.3 4.3-7.6 0.4-4.8

70.0 16.0 2.3 1.3 2.5 4.6 2.8

60.6 17.2 6.6 3.2 6.6 4.5 0.6

66.1 15.9 4.5 2.4 4.8 4.3 1.5

66.6 16.0 4.3 2.5 3.5

64.7

4.4 2.2

4.3 1.8 5.0 4.5 1.3

12

26

27

22

47

10

11

15.6

A. range of values. Gairloch acid gneisses (Park 1963); B, Shieldaig gneisses (Holland & Lambert 1973): C, Creag Mhor Thollaidh gneisses (Holland & Lambert 1973):D, Tollie central gneisses (Holland & Lambert 1973): E, Tollie SW gneisses (Holland & Lambert 1973):F, average tonalite (Rollinson & Fowler 1987).

CaO than the Tollie samples, but are otherwise similar. No rareearth element (REE) analyses have been carried out on the Gairloch gneisses but chondrite-normalized REE patterns in the Gruinard Bay samples vary in steepness, with some showing a marked Eu anomaly and others not (Rollinson & Fowler 1987) (Fig. 2.4A). Rollinson and Fowler inferred a range of original tonalitic liquid compositions, which they considered to have been produced by the partial melting of an amphibolitic basic source rock. The effects of several generations of intense deformation have obscured the relationships between the various protoliths of the Archaean gneiss complex. However several tentative conclusions can be drawn from the distribution of the main rock types. Granodioritic gneiss is the dominant rock in the SW and may have been derived from a large plutonic body. Tonalitic and granodioritic gneisses are interbanded in the area of the Tollie antiform suggesting that sheets of the one may have intruded the other; it is not possible to determine which is younger. The dioritic and gabbroic rocks of the eastern part of the area are clearly derived from a separate plutonic mass that appears to have been invaded by tonalitic material on its SW side.

2.2.2 Amphibolites The only geochemical data available for the early mafic and ultramafic rocks of the Gairloch district come from a small garnetrich amphibolite from the lalltaig gneisses analysed by Park (1966, table 1). This rock has a somewhat unusual composition (low in SiO2 and rich in Al2O3) and is not considered to be typical of the Gairloch early mafic rocks. However, Rollinson & Fowler (1987) present 12 analyses of early amphibolites from Gruinard Bay, which are petrographically similar to those of the Gairloch district and probably have the same origin. The Gruinard Bay samples are predominantly olivine-tholeiitic in normative composition. Rollinson & Fowler point out that the incorporation of the basaltic protoliths within tonalitic magma, their conversion to granulite facies and their subsequent retrogression to amphibolite facies would be expected to cause considerable disturbance to their chemistry. Nevertheless, the elements Ti, Zr, Y, and P retain good colinear trends reflecting a presumably igneous original composition. The main group of amphibolites is characterized by a wide spread of incompatible element values, and by flat to slightly steepened REE patterns (Fig. 2.4B). The broad uniformity of the incompatible element ratios is held to indicate that most samples were probably derived from the same mantle source region, although a small group may have been derived from a different source region, or may have undergone some crustal contamination. The similarity between the range of REE patterns found in these early mafic rocks and those of typical mid-ocean ridge basalts

(MORB) suggested that they represent disrupted fragments of ocean-floor basaltic crust. Whitehouse et al. (1996) distinguish two separate groups of early mafic bodies from the Gruinard Bay district, the first (their 'main amphibolite suite") broadly similar to the amphibolites described by Rollinson & Fowler (1987). but the second (their "hornblenditemetagabbro suite") characterized by light REE enrichment and large negative or positive Eu anomalies. They suggest that the second suite represents cumulates related to the tonalite-granodiorite magmatism. The metagabbroic components of the Creag Mhor Thollaidh hornblende gneisses of Gairloch probably belong to this latter suite.

2.3 Structural chronology The structure of a complex gneiss region such as Gairloch needs to be described in its chronological context. The period of time involved is very long, possibly from about 3.0 Ga to about 1.0 Ga. and the structural and metamorphic phenomena are divided into groups based on their perceived age relationships. However, the structural and metamorphic chronology on which the subdivision is based is itself founded on assumptions about ages and age relationships that have not been rigorously tested by modern precise geochronological studies. The overall geochronology of the Lewisian was summarized in Chapter 1.2, and only relevant aspects are reiterated here. The key to the structural chronology in Gairloch. as elsewhere in the Lewisian complex, is the Scourie dyke swarm, which, following Peach el al. (1907) and Sutton & Watson (1951). has been used to divide the complex into pre-dyke and post-dyke elements. However, none of the dykes attributed to the Scourie dyke swarm in the Gairloch area, as in the Southern Region generally, have been dated (except for K-Ar dates interpreted as metamorphic cooling ages). Therefore they may correlate with either the 2.4 Ga or 2.0 Ga dykes of the Central Region, or could even represent an as yet unidentified third swarm with an unkown age. It is also possible that dykes of two or more different ages are represented. Nevertheless, a suitable framework is needed in which the various structures can be discussed, and hence the traditional method of subdivision into pre-dyke and post-dyke structures is followed. As no evidence has been found in the Gairloch region of structures formed during the period of dyke emplacement, for example affecting some dykes and cut by others, it is considered that this approach is justified. Again, following Sutton & Watson (1951), all post-dyke structures are assigned to the Laxfordian. whilst recognizing the possibility that some structures affecting the dykes might not correlate with the type Laxfordian of the Central Region, thereby creating an anomaly in the classification. The predvke structures have been subdivided into an earlier Scourian set

CHAPTER 2

15

Fig. 2.4. Chondrite-normalized REE plots of Scourian tonalitic gneisses and amphibolites from Rollinson & Fowler (1987). (A) tonalites. plus hornblendite pod; (B) amphibolites.

16

THE EARLY STAGES - SCOURIAN AND I N V E R I A N

(D1 and D2) and a later Inverian set (D3) (cf. Park 1970). As explained in Section 1.2.5, the correlation of the Inverian with the type area in the Central Region is based on structural and metamorphic similarity, but there is no geochronological evidence to support it. The term 'Inverian' was introduced by Evans & Tarney (1964) for the metamorphic/structural event which produced steep NWtrending shear zones in the Assynt area, causing widespread retrogression of granulite-facies Scourian rocks to amphibolite facies and producing a set of new folds and associated fabric. Although similar in metamorphic grade and structural orientation to the Laxfordian, the Inverian structures were distinguished by being cross-cut and therefore post-dated by the Scourie dykes. Similar structures were described from Gairloch by Park (1964, 1965), Torridon (Cresswell 1972), and between Loch Maree and Gruinard Bay by Crane (1978). Park et al. (1987) considered that the Inverian event affected the whole of the Southern Region from the Gruinard river in the NE to Kenmore in the SW, although in many places, the Inverian structures are partly or completely obscured by the subsequent Laxfordian deformation.

Fig. 2.5. Simplified structure of the area around Loch Braigh Horrisdale, showing the anticlockwise rotation of N- to NE-trending Scourian foliation into the NW-trending Inverian 'straight belt*.

2.4 Scourian and Inverian structures Structures attributable to the Scourian and Inverian deformations can only be clearly distinguished from the younger Laxfordian structures in three areas within the Gairloch district: the area around Loch Braigh Horrisdale in the SW, the Creag Mhor Thollaidh area in the NE, and the Ard lalltaig block immediately south of Gairloch (see map and Fig. 2.1). In each of these areas, relict blocks or enclaves preserve the Scourian structure, which is characterized by a coarse gneissose banding with a NE-SW to north-south trend and steep dip. These enclaves are surrounded by younger, more highly deformed regions characterized by strongly developed NW-trending structures and fabrics of Inverian and Laxfordian age which are the dominant structures over the greater part of the district, and which obscure or obliterate pre-existing structures.

2.4.1 Braigh Horrisdale Scourian structures are dominant in a 2 km long, 500 m wide strip, west and south of Loch Braigh Horrisdale, towards the southwestern extremity of the exposed Lewisian of the Gairloch district, and also on the east side of the loch, immediately NE of the Braigh Horrisdale track. In this area, NW-trending, steeply-dipping, Scourie dykes, which are either undeformed or foliated only in narrow marginal zones, cut a series of coarse-grained, inequigranular, banded and massive gneisses with a NE-SW to north-south trend and a moderate to steep dip to the west. The massive gneisses are homogeneous in appearance but on close inspection reveal a preferred orientation of biotite flakes and flattened quartz aggregates that define the foliation. The banded gneisses contain a well-defined foliation consisting of mafic and felsic laminations and flattened lenses, as well as a marked mica and quartz-aggregate orientation. Thin pegmatite veins, sub-concordant to the foliation, occur in places. The mafic laminae have an average thickness of around 1 mm and the felsic laminae of about 3 mm, but there is considerable variation. The banded gneisses locally contain intrafolial folds to which the foliation is axial planar; these folds affect a pre-existing banding now transposed into the ubiquitous NE-trending foliation (Fig. 2.2). Since this older banding is the first structure to be identified in the Gairloch district, it is described as S1. In places, it is affected by a set of irregular, centimetre to metre scale, mesoscopic folds (F2) with NE-trending, steeply dipping axial planes parallel to the NE-trending foliation (S2) (Fig. 2.2); these areas are thought to represent the hinge zones of larger-scale, isoclinal, F2 folds. Both

banded and massive gneisses in places display a strong elongation lineation, which is parallel to, and thought to be coeval with, the Inverian lineation seen in the adjacent areas. The Braigh Horrisdale area appears to link southwards, via several inliers in the Torridonian cover, with the Ruadh Mheallan area in Torridon. described by Cresswell (1972) where similar structural relationships are seen. In their summary of the structure of the Southern Region of the Lewisian. Park et al. (1987) combine these two areas in the Ruadh Mheallan 'block' regarded as a Scourian enclave surrounded by younger Inverian and Laxfordian structures. In the Ruadh Mheallan area. Cresswell (1972) recognized two generations of folds and three generations of foliations pre-dating the Inverian structures. The earliest banding (thought to be of deformational metamorphic origin, rather than an original igneous layering) is affected by interference structures attributed to the superimposition of two fold phases (F2 and F3 in Cresswell's terminology). Since only the one set of folds (F 2 ) can be confidently identified in the Braigh Horrisdale area, these therefore correspond to F3 of Cresswell's sequence. However in view of the complexity of the Scourian deformational history in Torridon and also in Gruinard Bay (see Crane 1978) the Braigh Horrisdale gneisses may have been affected by earlier episodes of deformation, the evidence for which is now obscured. The gneisses exhibit an amphibolite-facies mineral assemblage, characterized by the association of calcic plagioclase and aluminous hornblende in the mafic rocks. Because of the relatively small size of the Scourian enclaves, and of the widespread presence of Inverian lineations, the whole area was presumably thoroughly recrystallized during the Inverian metamorphism: no relict Scourian metamorphic assemblages have been identified. The Braigh Horrisdale area is bounded on its NE side by a zone where the gneisses are strongly affected by small-scale folds with NW-trending, steep, axial planes and by a penetrative axial-plane foliation (Fig. 2.5). Over a distance of only a few metres, the NEtrending banding is rotated by these tight to isoclinal folds such that it is completely transposed into a NW-SE orientation. These younger structures are cross-cut by the Scourie dykes and are assigned to the Inverian deformation. This relationship can be matched in the Gruinard district on the northeastern side of the Southern Region (see Park et al. 1987) and in the Assynt type area, and is used to determine the Scourian age of the Braigh Horrisdale structures. The Inverian structures are well developed in a belt of ground, about 1 km wide and 3.5km long, immediately NE of the Braigh Horrisdale river. Two particularly good, accessible localities lie on

CHAPTER 2

17

Fig. 2.6. Inverian (F3) folds affecting Scourian S 1 /S 2 foliation in granodioritic gneiss, NE of Loch Braigh Horrisdale [804 705].

the NE side of a small loch at [809 704], about 700m east of the Braigh Horrisdale track, and at [802 710], 200m east of the track. At these localities, and elsewhere along the belt, the S1/S2 gneissose banding is affected by small- to medium-scale folds with steep, NWtrending axial planes. The folds vary considerably in style, from open to tight, or even isoclinal, and from sinusoidal or ptygmatic to angular and chevron-like. They vary from millimetres to metres in wavelength, depending mainly on the thickness and composition of the folded layers, which, because of their complex previous history, are themselves highly variable. A typical example is shown in Figure 2.6. In some places, a new penetrative fabric is developed, which has both planar and linear elements, marked by the alignment of micas and flattened or elongate quartz aggregates. The lineation has a steep plunge, parallel to the S2/S3 intersection. Many of the folds are asymmetrical, with attenuated middle limbs, and all gradations are seen between asymmetric folds and minor shear zones. Where the folds are tight to isoclinal, the S1/S2 banding has been rotated on the F3 fold limbs and transposed into a NWSE trend with a steep dip, sub-parallel to S3. Northeast of a line from [800 713] to [816 693], the belt of Inverian folding is replaced by a belt of regular foliation with a uniform northwesterly strike (average trend about 125°) and steep dip. This Inverian 'straight belt' is a larger-scale counterpart of the narrow Inverian shear zones seen at Ard lalltaig (see below and Fig. 2.8). The S1/S2 foliation has a more north-south trend immediately SW of the intensely folded belt, and the axial planes of the folds have trends, which although highly variable, are typically clockwise with respect to the trend of the foliation in the straight belt. The margin of the straight belt is also oblique, in an anticlockwise sense, to the foliation in the straight belt. This pattern is suggestive of a sinistral sense of shear on the main shear zone (Fig. 2.5). The straight belt extends to the margin of the Gairloch supracrustal belt near Shieldaig, and is about 1.3 km wide. However the northeastern part of the belt is strongly affected by Laxfordian deformation (see Section 6.2.2).

2.4.2 Creag Mhor Thollaidh The area between the Leth-chreige crush belt in the west and Loch Maree in the east (Fig. 2.1), which is about 4 km long and up to 1.5km wide, was termed the Creag Mhor Thollaidh block by Park (1970), and is characterized by weakly deformed and undeformed Scourie dykes. There is an enclave of older Scourian structure centred on the summit of Creag Mhor Thollaidh, where F3 (Inverian) folds, together with S3 foliation and L3 lineation, are superimposed on the Scourian S1/S2 gneiss fabric, which has preserved a generally NESW strike and steep to moderate dip to the SE. The Creag Mhor Thollaidh enclave has the shape of an asymmetric augen, about 1.3 km long and 1 km across (see map and Fig. 3.3). It is surrounded on three sides by NW-trending, steeply-dipping gneisses affected by the Inverian deformation, and in the north by the Tollie Farm fault. The summit area of Creag Mhor Thollaidh, around [865 775], is composed of NE-trending gneisses dipping steeply (60-70°) to the SE, which display a prominent lineation that plunges down the dip of the foliation. Both homogeneous and banded gneisses are present, similar structurally to those in the Braigh Horrisdale area, but consisting almost entirely of quartz-feldspar-hornblende (metadiorite) gneisses. The banding is affected by small- to medium-scale folds with axial planes striking NE-SW and dipping steeply SE, and by a penetrative axial-plane foliation. By analogy with the Braigh Horrisdale outcrops, these structures are respectively assigned to F2 and S2. The S2 banding is locally affected by mesoscopic folds (F3) with NW-striking, steeply dipping, axial planes and a penetrative axialplane foliation (S3). Both folds and foliation are considered to be Inverian in age, as they are similar in orientation and appearance to the Inverian structures surrounding the augen. The intersection lineation (L3) associated with the NW-trending foliation is parallel to the steep lineation described above, which occurs in areas unaffected by the Inverian microfolds. Like the Braigh Horrisdale area, therefore, this area is thought to have been affected by Inverian metamorphism and deformation throughout. The F3 folds

18

THE EARLY STAGES - SCOURIAN AND INVERIAN

Fig. 2.7. Interference between NE-trending Scourian F2 folds and NW-trending Inverian F3 folds, Creag Mhor Thollaidh. near summit of hill [863 777]; coin is 3 cm in diameter.

here are generally small-scale (under 1 m in wavelength) and tend to be more symmetrical than is the case in the Braigh Horrisdale area. Small-scale interference structures result from the intersection of F2 and F3 folds (Fig. 2.7). At the margins of the enclave, the foliation assumes a northwesterly trend with a steep dip. The transition between areas where S2 is dominant to where S3 is dominant can be seen at [865 774], where the folded S2 is replaced southeastwards by uniformly NWtrending S3. As in the Braigh Horrisdale area, the S3 foliation is partly transposed S2 banding, rotated and transposed parallel to 83, and partly new penetrative S3 fabric. In the east and SE of this part of the block, folds are uncommon, partly due to the homogeneous nature of much of the gneiss. Where folds do occur, they are mostly small and intrafolial, and it is unclear whether they are Inverian or older. The asymmetric shape of the enclave (see Fig. 3.3), together with the anticlockwise bending of S2 near the margins, are consistent with an overall sense of sinistral shear across the enclave, but there is no field or petrographic evidence of shear sense to support this.

2.4.3 The lalltaig block The third area where Scourian structures are well preserved is on the peninsula of Ard lalltaig, 1 km south of Gairloch (Fig. 2.8B). Here, Scourian gneisses occupy a wedge-shaped area, 1.6 km long and 500 m wide, bounded by steep, NW-trending shear zones which separate the gneisses from adjoining rocks of the Loch Maree Group (see map). The structures are most easily studied on the wellexposed rocky crags at the NW end of the peninsula. A small part of this area has been mapped in detail at a scale of 1:1200 by Park (1963, 1964) (Fig. 2.8A). This small area [804 735] is bounded by steep, north-trending shear zones, and is also transected by earlier, NW-trending shear zones, a few metres in width. Between these shear zones, a coarse gneissose banding strikes NE-SW and dips at 40-60° SE. Again, by analogy with the Braigh Horrisdale area, the gneissose banding is assigned to S2 (or, strictly, S1 transposed into S2). The banding is quite coarse, 1 mm to 1 cm in scale, very irregular in places, and encloses lenses and irregular masses of mafic and ultramafic material up to several metres in diameter and also lensoid felsic

pegmatite veins. Minor F 2 folds with axial planes parallel to this banding are extremely variable in style and orientation. Two larger, mesoscopic, examples can be seen in Figure 2.8A. In contrast to the gneisses of the Braigh Horrisdale and Creag Mhor Thollaidh areas described above, the gneisses here contain granulite-facies assemblages, variably retrogressed to amphibolite facies. The granulite-facies assemblage consists of garnet, clinopyroxene, orthopyroxene, brown hornblende, plagioclase. and scapolite and is retrogressed to an assemblage characterized by bluegreen hornblende and epidote, where the large feldspars are recrystallized to smaller, clear, polygonal grains. The gneisses, though very variable in composition, are generally much more mafic than those of the Braigh Horrisdale area and their abundance of garnet also distinguishes them from the Creag Mhor Thollaidh gneisses. The narrow, NW-trending. steep shear zones that cut the S2 banding are considered to be of Inverian age (i.e. S 3 ). They are characterized by retrogression of the granulite-facies assemblage to amphibolite facies, accompanied by a reduction in grain size. Lineations are usually steeply plunging, in places parallel to the intersection of S2 and S3. In contrast, the younger shear zones, which include the two bounding shear zones shown on Figure 2.8, have a flaggy, fine-grained appearance, with a strongly developed lineation plunging at 30-40° to the NW. These shear zones are mylonitic in places, and involve retrogression to greenschist facies. They are considered to be Laxfordian D3 in age (see Section 5.4) and include the two shear zones bounding the lalltaig block on its SW and NE sides; these boundary zones are up to 100m wide.

2.5 Metamorphism Relict Scourian granulite-facies assemblages are only found in small parts of the Creag Mhor Thollaidh and Ard lalltaig blocks where they are partially retrogressed to amphibolite-facies assemblages (see above). Elsewhere, in these areas little affected by Laxfordian deformation, the metamorphic assemblage exhibited by the Archaean gneisses is characterized by the association of calcic plagioclase and blue-green aluminous hornblende in metabasic rocks, indicating amphibolite-facies metamorphic conditions. These amphibolite-facies assemblages are associated with Inverian

CHAPTER 2

19

Fig. 2.8. Structure of the Ard lalltaig peninsula. (A) Detailed map of part of the peninsula showing S1 foliation generally transposed into NE-trending S2, with several F1 folds, affected by NW-trending and younger north-trending shear zones. (B) Map of the Ard lalltaig peninsula showing the location of Figure A and the main shear zones.

fabrics. However the Scourie dykes are also thoroughly metamorphosed in amphibolite fades (Section 3.2); thus amphibolite-facies Laxfordian metamorphism must have affected the whole of the Gairloch district, not just those parts affected by Laxfordian deformation. In the absence of the appropriate geochronological data on the gneisses, it is not possible to determine the extent to

which the gneisses in the Inverian enclaves experienced recrystallization during the Laxfordian event. The structural evidence suggests that pervasive recrystallization may have been confined to narrow zones such as the dykes and certain minor shear zones, perhaps aided by the availability of fluids.

Chapter 3 The Scourie Dyke Suite The Scourie dyke suite has only been securely dated in the Central Region, and no emplacement ages have been published from the dykes in the Southern Region. In view of the possibility that the Central and Southern Regions may not have coalesced until after the dyke emplacement, the equivalence of the swarms in the two regions has not been proved. Nevertheless, the dyke swarm in the Southern Region has traditionally been referred to as the 'Scourie dykes' and for convenience, this nomenclature is adopted here.

3.1 Distribution and field relations Amphibolite bodies attributed to the Scourie dyke suite are abundant within the Scourian gneiss complex of the Gairloch district. Several sill-like amphibolite sheets of doubtful affinity occur within the Loch Maree Group; however, these are chemically distinct from the Scourie dykes (see Section 4.9.2) and no bodies that can be identified as Scourie dykes cut the rocks of the LMG. The dykes weather dark grey to almost black and contrast sharply with the much paler acid gneisses. They are generally sheetlike in form, although many show considerable irregularity of shape. The igneous origin and intrusive relationship of these bodies to the host gneisses can be widely demonstrated, and are particularly clearly displayed in the eastern part of the Tollie area, for example on Creag Mhor Thollaidh, and in the southwestern part of the district, around Loch Braigh Horrisdale. In both these areas, the dykes are grossly discordant to the banding of the host gneisses (see map). However, in the two belts of gneisses adjoining the LMG outcrop (a 2 km-wide belt on the NE side, corresponding to the SW limb of the Tollie antiform, and a rather narrower belt on the SW side, south of Shieldaig) the dykes are generally broadly concordant and sill-like in relation to the foliation in the host gneisses, which has roughly the same orientation. Within these belts the effects of Laxfordian deformation have largely obscured any original discordances. Nevertheless, with careful observation, it is normally possible to demonstrate local discordance at the margins of dykes even in the most highly deformed areas, and thus to prove their intrusive nature. Even apparently concordant amphibolite sheets show evidence of local discordance, when their margins are followed for some distance and examined closely.

3.1.1 SW of the LMG belt The Gairloch district exhibits the greatest concentration of Scourie dykes in the whole Lewisian complex. Immediately SW of the LMG outcrop, this concentration reaches its maximum expression, with over one-third, and in places almost half, the outcrop area being occupied by dykes. This belt of extreme dyke concentration is about 2 km wide; its margin to the NE is marked by a line extending SE from Shieldaig Lodge and to the SW by the track leading SE from Loch Braigh Horrisdale. Individual dykes, which generally trend NW and have steeply dipping margins, exhibit very complex branching shapes, bifurcating and rejoining, and enclosing numerous screens of the host-rock gneisses (e.g. Fig. 3.1). The dykes in this belt are typically deformed, particularly along their margins, and the whole belt has been affected by Laxfordian deformation. Evidence from the shapes of deformed mafic or felsic grain aggregates, which can be used as strain markers in certain dykes, indicates that in general the dykes have been thinned and extended during this deformation (Park et al. 1987). However examination of the margins leaves no doubt that the outcrop shapes are not merely the product of Laxfordian folding and high strain, but represent

Fig. 3.1. Complex branching intrusion shapes of Scourie dykes SW of the LMG belt. A, immediately NE of Loch Braigh Horrisdale, grid squares [80 70] and [81 70]; B, Sidhean Mor, grid squares [80 71] and [81 71].

originally intrusive forms. Branching occurs in both the vertical plane as well as the horizontal, since abrupt changes in outcrop pattern are seen across comparatively minor normal faults. The dykes in this belt are generally concordant or near concordant with the foliation in the host gneisses although again, minor local discordances confirm their intrusive relationship to the Inverian foliation. Dyke widths vary considerably. Although the thickest dykes are locally over 100m in width, there are numerous thin dykes between 1 m and 10m wide, many too thin to be mapped, and the mean width is probably less than 20m. Southwest of Loch Braigh Horrisdale, the dykes, although still numerous, occupy only about 20% of the outcrop area. Dyke widths in this belt vary from 10m to 50m and the individual dykes are straighter, more regular and generally thicker than those to the NE, cutting across NE- to north-trending Scourian banding. Only in a few cases, do minor shears along and parallel to the margins create local concordance. Gross discordance is seen SW of a line from [790 713] to [817 693]. Park & Cresswell (1972, 1973) drew attention to the relationship between dyke frequency and thickness, and the structure of the host gneisses, particularly in the Torridon area, where it was noted that dykes were thinner and more frequent in belts of NW-trending, steep, Inverian foliation compared with those in areas of north- to NE-trending Scourian foliation, where they were thicker and more widely spaced. The closely foliated Inverian structure may have provided more convenient, suitably orientated, pathways for the dyke magma which was able to exploit the structure by forming numerous parallel and linking fissures along the weaknesses created by the foliation.

22

CHAPTER 3

Fig. 3.2. (A) Discordant Scourie dyke margin cutting Scourian foliation, west of Loch Braigh Horrisdale, at [805 705]; (B) Scourie dyke margins, generally concordant with Inverian foliation, but showing minor discordance [816 713].

Figure 3.2 shows examples of both types of marginal relationships, taken from two localities 1 km apart on the NE side of Loch Braigh Horrisdale. Figure 3.2A illustrates a grossly discordant dyke margin against the NE-trending S2 foliation at the SW margin of the zone of intense Inverian folding. Figure 3.2B illustrates a typical relationship in the belt of generally concordant dykes within the Inverian shear zone. The local discordance at the margin of this narrow dyke branch is very clear.

3.1.2 NE of the LMG belt The original relationships between the dykes and the host gneisses can be clearly seen east of the Leth-chreige crush belt, in a 1.5 kmwide belt extending from Tollie Farm in the NW to Loch Maree in the SE (the Creag Mhor Thollaidh block; see map). Here the dykes are generally concordant or sub-concordant with the dominant foliation, but the marginal relationships are little affected by Laxfordian deformation and small discordances are readily observed. In this belt, dykes occupy about 25% of the outcrop area, with an average thickness of between 30 and 40m. The dykes exhibit some branching, but are generally regular, steeply dipping, parallelsided bodies trending NW, except around Creag Mhor Thollaidh, where they bend gradually into a north- to NE-trending orientation, and in the west, along the east side of the Leth-chreige valley, where they are affected by a north-trending fold. West of Tollie Farm, the dykes swing back into their more normal NNW trend. The swing in strike from NW to NE at the summit of Creag Mhor Thollaidh corresponds to a change in the dominant hostrock strike from NW to NE. In an area of about 1 km2 south of Tollie Farm, NE-trending Scourian banding dipping steeply SE is preserved, and only locally affected by the younger NW-trending Inverian and Laxfordian folding, whereas to the SE, the Inverian trend is dominant. In contrast to the Loch Braigh Horrisdale area described above, where the Scourie dykes maintain their NW-SE

Fig. 3.3. Pattern of Scourie dykes at and around the summit of Creag Mhor Thollaidh. The dykes are generally concordant with the dominant Inverian foliation, which envelopes a low-strain Scourian enclave. Unusually, the dykes within the enclave strike NE. parallel to the Scourian foliation.

THE SCOURIE DYKE SUITE

23

Fig. 3.4. Discordant Scourie dyke cutting gently-dipping Inverian foliation near the crest of the Tollie antiform, south of Loch Tollie, at [845 776]. Both the dyke and the Inverian foliation have been rotated into their present attitude by intense Laxfordian D2 deformation, which has resulted in a mainly linear fabric in the dyke.

orientation and cut discordantly across the Scourian banding, here the dykes appear to be deflected around an augen-shaped mass of Scourian structure (Fig. 3.3). The thin concordant NE-trending dykes crossing this augen may represent minor fissures linking larger NW-trending dykes on each side. Between the Leth-chreige valley and the LMG outcrop, there is a somewhat different picture. Here, Laxfordian deformation has been more intense and pervasive, resulting in the dykes being folded around the Tollie antiform. The dykes here are highly deformed, and their outcrop shapes are modified to a much greater degree by subsequent deformation than SW of the LMG. On the NE limb of the Tollie antiform, the dykes dip moderately to steeply to the east or NE. They exhibit a strong linear fabric which plunges to the SE at angles varying from 0-15° in the north to around 70° in the south (see map). The outcrop pattern shows considerable branching and in areas of low dip is affected by the topography, and locally exaggerates the dyke thickness. Because of the high strain shown by these dykes, which have been thinned, and extended in a northwesterly direction, the apparent widths probably seriously underestimate the original thicknesses. The spacing is considerably greater than in the Creag Mhor Thollaidh area to the east, as the dykes only occupy some 10 to 15% of the outcrop area. The dykes are broadly concordant with the gneissose foliation. This concordance is caused mainly by the intense Laxfordian deformation; however local discordances with the originally steep, pre-dyke gneissosity are still clearly evident (Fig. 3.4). The outcrop shape of successive dykes can be used to mark the trace of the Tollie antiform southeastwards from Loch Tollie. On the SW limb of the antiform, the dykes rapidly steepen southwestwards to near vertical within about 1 km of the fold axial trace. On this limb, the dykes are thinner, and more affected by deformation. They lie within the Gairloch shear zone (see Section 6.4.2), which has resulted in further thinning and extension of the dykes, and the development of minor, upright satellite folds related to the main Tollie antiform. Many of the thinner dykes or dyke branches cannot be represented on the map, and the average dyke thickness in this belt is probably less than 10m. However the spacing is similar to that on the NE limb, the dykes occupying around 10-15% of the outcrop area.

The effects of intense Laxfordian deformation generally obscure the original marginal relationships of the dykes on the SW limb of the antiform. However on the NE limb, although both Inverian foliation (S3) and dykes have been rotated by Laxfordian deformation into a gentle to moderate inclination, the marginal relationships are normally still clear when viewed down-plunge. The relationships are particularly well displayed in the crags immediately south of Loch Tollie, at [845 776], Here, gently inclined dykes are seen cutting moderately-dipping to subhorizontal S3 at an angle of about 20° (Fig. 3.4). Elsewhere on this limb of the fold, the dykes are generally concordant with S3 but discordant sections, where dykes are locally steeper than S3, can frequently be observed, again looking down plunge. Viewed at right angles to the plunge direction (i.e. in the approximate plane of X and Z of the Laxfordian strain ellipsoid) any angle of discordance is much reduced by the effect of the high strain and is difficult to detect.

3.2 Petrography The Scourie dykes of the Gairloch district are all in the form of amphibolite, having experienced pervasive amphibolite-facies Laxfordian metamorphism. Only one dyke has been examined in which the original igneous texture and mineral assemblage are both still clearly evident. This dyke is located at [886 758] on the Rubh Aird an Anail peninsula, on the shore of Loch Maree (see map). It is about 13m wide at this point, but becomes wider to the NW. The dyke is undeformed, and exhibits distinct chilled margins against the metadiorite gneiss. The original lath-shaped plagioclase crystals are largely preserved, and the surrounding mafic material consists of clinopyroxene grains partly replaced by hornblende-quartz aggregates, with distinct rims of hornblende surrounding the feldspars. Garnet crystals envelop both feldspar and clinopyroxene. Elsewhere, the original igneous mineralogy has been completely replaced, although in many dykes, traces of a relict sub-ophitic texture are preserved. This relict igneous texture is particularly evident in the central parts of a number of very thick dykes, for example the dyke making up the summit ridge of Sidhean Mor [812 715], and may appear very coarse, i.e. gabbroic rather than

24

CHAPTER 3

Fig. 3.5. (A) Unfoliated coarse metadolerite, showing relict sub-ophitic texture. The mafic areas are composed of hornblende aggregates and the felsic areas of plagioclase, quartz and epidote. Sidhean Mor [820 716]; width of field, 13mm ( x l 0 ) . (B) Well-foliated, fine-grained, completely recrystallized metadolerite. consisting mainly of hornblende and plagioclase. SW of Loch nam Buainichean (861 730); width of field. 14 mm (x7.5).

doleritic, with lath-shaped feldspar aggregates reaching 5mm or more in length (Fig. 3.5A). This 'coarse' grain size is a relict feature, the crystals of the replacive grain aggregates being much smaller than the original grains. Hornblende is the most abundant constituent of the amphibolite dykes, forming between 50% and 75% of the rock in the mafic amphibolites, and up to 95% in the ultramafic varieties. Plagioclase is the other main component. Minor constituents are quartz (up to about 10%) and biotite, and in some dykes, garnet, epidote and scapolite form significant proportions. Accessory minerals are apatite, titanite and opaque ore, including both magnetite (in some cases altered to haematite) and ilmenite. The mineralogy is dependent to some extent on the degree of alteration of the original igneous assemblage, which in turn depends partly on the pervasiveness of the deformation. In the least-altered dykes (e.g. the central undeformed portions of the Sidhean Mor dyke referred to above) the shapes of the original lath-shaped feldspars can easily be recognized, but they have been recrystallized to polygonal aggregates of plagioclase grains. These lath-shaped relics are surrounded by mafic aggregates that consist of large hornblende grains, sieved by small quartz and feldspar inclusions, which

pseudomorph the original clinopyroxene. These large grains are surrounded by rims of smaller prismatic hornblendes. In some cases, large interlocking prismatic hornblende crystals intersect the lath-shaped feldspar aggregates. Garnet porphyroblasts are common in the dykes of the southwestern belt, between Shieldaig and Loch Braigh Horrisdale. However they are very unevenly distributed, being found only in some of the dykes, and. in many cases, only in certain bands within these dykes. These porphyroblastic garnets are typically rimmed by. and partly intergrown with, plagioclase. which in some cases has been partly replaced by epidote or scapolite. This type of garnet has a quite different mode of occurrence to the smaller garnets of the least-altered dyke, which appear to be earlier. Ilmenite is characteristic of the incompletely recrystallized dykes, and is typically rimmed by sphene. In completely recrystallized dykes, sphene co-exists with magnetite. Most of the dykes, although varying considerably in degree of metamorphism and deformation, are broadly similar petrographically, and belong to the suite known traditionally as 'epidiorites' (see Peach et al. 1907). The members of this suite are also chemically similar and vary mainly in mafic felsic ratio. However

THE SCOURIE DYKE SUITE

25

Table 3.1. Chemical analyses of Scourie dykes from Gair loch

SiO2 TiO2 A12O3 Fe2O3 FeO MnO MgO CaO Na2O K26 P2O5

Ga Cr V Li Ni Co Sc Zr Cu Y Sr Ba Rb n

A

B

C

D

E

49.2 1.34 14.3 4.6 9.7 0.08 6.1 9.0 3.2 0.85 0.12

50.8 0.67 14.0 2.3 9.1 0.15 7.8 11.2 1.8 0.58 0.07

50.5 1.47 13.5 3.9 10.3 0.21 5.5 8.7 2.5 1.2 0.14

50.2 1.16 13.9 3.6 9.7 0.15 6.5 9.6 2.5 0.88 0.11

49.5 1.12 14.2 2.4 10.0 0.17 6.9 9.8 2.4 0.30 0.1

17 116 289 12 82 38 67 120 139 38 267 259 19

17 198 249 94 47 55 18 159 119 19

20 94 342 87 111 167 26 207 179 29

18 136 293 87 93 120 72 211 186 22

18 159 315 96 73 82 24 149 75 7

6

15

7

28

28

A, mean of six Scourie dykes, from Park (1966); B, mean of 15 dyke analyses from two dykes, from the Shieldaig area, from Johnson et al. (1989); C, mean of seven dyke analyses from the Tollie area, from Johnson et al. (1989); D, mean of 28 Gairloch Scourie dyke analyses (A, B and C); E, mean LMG amphibolite (from Table 4.5). N.B. Standard deviations for B and C are given in Johnson et al. (1989).

there are unusual minor variants which have been recognized in the field. Ultramafic types can be distinguished easily by their dark green, almost black, appearance at outcrop. They contain at least 90% hornblende, with minor biotite, plagioclase, and opaques. These dykes are considered to be the metamorphosed equivalents of the picrite dykes of the Assynt district (e.g. see Peach et al. 1907, Tarney 1963). A good example of one of these dykes occurs on the SW margin of an amphibolite dyke 300 m NW of the summit of Mullach nan Cadhaichean at [825 696]. The ultramafic dyke here is 1-2 m wide and is veined by the adjacent metadolerite. Another variant is distinguished by the presence of large rectangular relict phenocrysts of plagioclase instead of the more usual sub-ophitic texture. One such dyke occurs 200 m SW of the large Sidhean Mor dyke, at [807 717]. Massive, originally coarse-grained, dykes, which are either undeformed, or foliated only in narrow zones, especially along their margins, are typical of the southwestern part of the district, around and south of Loch Braigh Horrisdale, and also in the Creag Mhor Thollaidh belt in the NE. However, examples occur also in the highly deformed belts; for example, in the SW limb of the Tollie antiform, a massive undeformed dyke can be traced for over 1 km on the NE shore of Loch Arigh Mhic Criadh, at [830 767], where it varies from 10 to 30m in width. The deformed dykes show varying degrees of foliation development from completely unfoliated to highly schistose, but generally they do not become uniformly schistose like the amphibolites of the LMG, and in many cases the fabric is weakly developed in comparison with the adjoining gneisses. The foliation comprises both a grain-aggregate shape fabric and a preferred orientation of prismatic hornblende crystals (Fig. 3.5B). The shape fabric provides a convenient strain indicator and has been used to quantify the variation in Laxfordian strain throughout the Gairloch district and

Fig. 3.6. MgO v. Zr (A) and TiO2 v. Zr (B) plots of Gairloch Scourie dykes showing distinct grouping of Sithean Mor and Tollie dykes (see text)

adjoining areas (see Park et al. 1987; Niamatullah & Park 1990). The fabric varies from strongly linear (L) in the crest and NE limb of the Tollie antiform, to strongly planar (S) in the SW limb. Elsewhere, intermediate (LS) fabrics are more typical. The grain size of the dykes varies considerably, but less so than in the LMG amphibolites. The thin dykes in the SW limb of the Tollie antiform are generally finer-grained (typically around 0.1-0.2 mm) than elsewhere. The coarsest amphibolites, with a grain size of up to 5-10 mm, are found in completely recrystallized dykes in the crest and eastern limbs of the Tollie antiform. Southwest of the LMG belt, the grain size varies from 0.2-0.3 mm in the well-foliated varieties to around 0.5mm in the coarser-grained unfoliated dykes. A similar range is evident in the Creag Mhor Thollaidh belt. As noted above, the metamorphic grain size of the massive dykes with coarse relict texture is actually much finer than the original igneous grain size.

3.3 Chemistry and origin Twenty-eight analyses including both major and trace-element data are presented in Table 3.1 for the Scourie dykes of the Gairloch area, comprising 11 from dykes in the Tollie gneisses and 17 from

26

CHAPTER 3

Fig. 3.7. Multi-element profiles of three Scourie dyke samples from Gairloch (A) compared with three Scourie metadolerite dykes from the Central Region (B) (from Park et al. 2001).

Fig. 3.8. Rock/chondrite REE plots of Scourie dykes: (A) Scourie dyke from Gairloch, from Johnson (1988); (B) seven Scourie dolerite dykes from Assynt, from Weaver & Tarney (1981).

four separate dykes in the Shieldaig gneisses (Park 1966; Johnson et al. 1987; Johnson 1988). Of the 17 dyke analyses from the Shieldaig gneisses, five are from the thick Sidhean Mor dyke and 10 from a thinner dyke to the NE. Both the Tollie and Shieldaig dykes possess a basaltic chemistry and resemble the widespread early quartz-dolerite suite of the Central Region (e.g. Tarney & Weaver 1987b). They show a greater range in composition than the group A amphibolites of the LMG but are comparable in many respects to the group B amphibolites (see Figs 4.13, 4.14). Significant differences are evident between the Tollie and Shieldaig dykes sampled (Table 3.1). The Tollie dykes have higher TiO2, K 2 O and Zr than the Shieldaig dykes, and lower MgO and CaO, suggesting that they are more evolved (see Johnson 1988 and Fig. 3.6). The Tollie dykes have a similar composition to average continental basalt, but with slightly depleted A12O3 and MgO, and slightly enhanced K 2 O and TiO 2 . indicating that they are moderately evolved, whereas the Shieldaig dykes exhibit a more primitive composition. Incompatible elements Ti, P. and Y all show strong positive correlation with Zr indicating a fractionation trend (see Johnson 1988 and Fig. 3.6B). Both the Tollie and Shieldaig dykes fall on the same trend, with the Tollie dykes showing generally higher values. Multi-element profiles for three samples (Fig. 3.7) are similar to those of the Scourie metadolerite dykes of the Central Region. Two samples display a negative Nb-Ta trough. Rare-earth-element data for one sample from the Tollie group (Fig. 3.8A) has a Ce/Yb N ratio of 2.944 and a La/Sm N ratio of 1.853. which denotes a fractionation pattern indicating moderate to high LREE enrichment. This pattern is similar to those shown by the quartz-dolerite suites of the Central Region (Fig. 3.8B) (e.g. Tarney & Weaver 1987b), and is typical of a continental tholeiite. The general similarity between the Scourie dykes of the Central Region and the amphibolite dykes of the Gairloch area suggests that the latter could well belong to the same suite and may have originated in the same way, although they may have belonged to a different continental plate prior to Laxfordian collision (see Section 7.3). There is some doubt as to whether the less evolved Shieldaig

THE SCOURIE DYKE SUITE

dykes to the SW of the LMG belt belong to the same suite as the Tollie dykes. However, the geochemical data for the Shieldaig dykes cover such a small number of dykes, and such a narrow geographical range, that meaningful comparisons with the Central Region dykes or speculations about a separate origin are premature. The origin of the quartz-dolerite suites of the Central Region is discussed by Tarney & Weaver (1987b). The trace-element data indicate that the mantle source of the magmas that supplied the quartz dolerites was enriched in large-ion-lithophile (LIL) and light rare-earth elements. Weaver & Tarney (1981) suggest that this enrichment may have occurred at the time when the gneisses were

27

generated, and that crustal components from the subduction zone, enriched in LIL and with a prominent negative Nb anomaly, contaminated the overlying mantle wedge, becoming part of the sub-continental lithospheric mantle. Another possibility considered by Tarney & Weaver (1987b), following Oxburgh & Parmentier (1978), is that refractory harzburgitic mantle material resulting from the extraction of basalt at an ocean ridge may, because of its buoyancy compared with fertile mantle, rise diapirically from the subduction zone to underplate the continental lithosphere. This material would be a potent source of heat, causing melting of the phlogopite- or hornblende-rich sub-continental mantle to produce voluminous tholeiitic magmas.

Chapter 4 The Loch Maree Group The supracrustal rocks of the Loch Maree Group (LMG) consist of a variety of metasedimentary rocks interbanded with amphibolites considered to be of volcanic origin. The metasedimentary rocks fall into two distinct categories: a) schistose semipelites, which form the main part of the outcrop; and b) narrow bands of different rock types, including siliceous, carbonate-bearing and graphitic rocks, occurring in close association with the metavolcanic amphibolites. Both the compositional banding and the dominant foliation throughout the LMG outcrop are steeply dipping and trend uniformly NW-SE. The sequence of lithotectonic rock units from SW to NE (structurally upwards) is shown in the cross-section (Fig. 4.1) and briefly described in Table 4.1. The original names of the lithotectonic units (Park 1964) are retained for convenience. The depositional age of the LMG is presumed to be around 2.0 Ga, based on a Sm-Nd model age (O'Nions et al. 1983) and detrital zircon dates (Whitehouse et al. 1997a, 2001) (see below).

4.1 Semipelites 4.1.1 Distribution Semipelites form several distinct NW-trending belts separated by amphibolite sheets. The most prominent belt comprises the Flowerdale schist unit (see map) which occupies a broad belt about 700 m in width, extending in a northwesterly direction across the Gairloch district, but ending north of the mapped area, where the two amphibolites from either side converge, 3.5 km north of the Gairloch-Poolewe road. This belt is offset in the centre of the area by the Flowerdale fault, and has a total exposed length of about 15km. Southwest of this belt is the Kerrysdale schist unit which occupies a narrower belt, up to 200 m in width, enclosed within the Kerrysdale basite (amphibolite) sheet. Narrower bands of Kerrysdale-type semipelite occur within the amphibolite in the area between Kerrysdale and Loch Bad an Sgalaig. The main Kerrysdale schist unit wedges out near Flowerdale House, at [813 753], where the two main branches of the Kerrysdale basite unite. Southwest of the Kerrysdale basite, another semipelite unit, the Charlestown schist, can be traced from near the Gairloch Hotel in

the NW, at [807 760], to Dubh Loch in the SE. Other narrow bands of schistose semipelite occur: (1) within the outcrop of the Ard gneiss unit, extending from Loch Kerry SE to Dubh Loch; (2) within the Mill na Claise basite; and (3) on the NE side of the Aundrary basite unit, within the Creag Bhan crush belt. The schistose semipelites typically form smoother and less wellexposed outcrops compared with the adjoining rugged amphibolite outcrops, which tend to form prominent rocky ridges. The Flowerdale and Charlestown semipelites are similar in appearance and composition, whereas the Kerrysdale semipelite is appreciably more siliceous.

4.1.2 Petrography Typical semipelites of the Flowerdale and Charlestown units are fine-grained, schistose, uniform-textured rocks, weathering brown, but with dark grey to almost black fresh surfaces (Fig. 4.2A). They contain 30-40% quartz, 20-30% plagioclase (oligoclase) and between 30 and 40% reddish-brown biotite. Muscovite occurs in small amounts in some samples, and epidote, calcite and opaque ores (typically pyrite), occur in minor quantities, in most. Small garnets are found locally in the Charlestown schist but are absent in the Flowerdale schist. Green chlorite may occur partially replacing biotite, but a colourless chlorite also occurs as part of the stable assemblage. Apatite and tourmaline are present as accessories. Quartz and plagioclase form polygonal equigranular mosaics bounded by straight edges of aligned biotite laths, which occur either in bands, or scattered throughout the rock. The southwestern part of the Flowerdale schist outcrop is characterized by a variety of semipelite containing appreciable quantities of muscovite and calcite, whereas in the main part of the outcrop, these minerals are generally absent. The semipelites of the Kerrysdale schist unit are similar in many respects to the Flowerdale and Charlestown varieties but their more siliceous character is normally evident at outcrop, where the rock is appreciably harder, with a black 'flinty' appearance on fresh surfaces, and in many places is marked by prominent quartz stripes and lenses. The quartz content is much higher and the plagioclase and mica content correspondingly lower. Typical samples contain

Table 4.1. Lithotectonic rock units of the Loch Maree Group (SW to NE) (Width of outcrop given in metres) Cloiche marble belt: highly deformed granodiorite gneiss (similar to Ard gneiss, below) and amphibolite sheets with thin quartz-biotite-garnet schist and marble (meta-limestone) bands; 400 m. Mill na Claise belt: amphibolite sheet enclosing three narrow bands of mixed semipelite, magnetite quartzite and graphitic pelite, surrounded by a narrow outcrop of semipelite schist; in places gneissose, with granodiorite gneiss (Mill na Claise gneiss) enclosing thin amphibolites in the fold core; up to 800 m. Ard gneiss: granodiorite to tonalite gneiss, enclosing several amphibolite sheets; up to 500 m. Charlestown schist: quartz-biotite-plagioclase schist (semipelite) with narrow biotite-amphibolite and amphibolite bands; 100-300 m. Kerrysdale basite: amphibolite sheet containing bands (several metres wide) of quartzite (metachert?), striped magnetite quartzite (banded iron formation), biotite-garnet-grunerite schist and graphitic quartz-muscovite schist (pelite), and also several wider bands of highly siliceous quartz-biotite schist (semipelite), the thickest 150 m wide (all these bands lens out laterally); 350-500 m. Flowerdale marble belt: quartz-chlorite schist with narrow (up to a few metres wide) bands of striped magnetite-quartzite (banded iron formation), graphitic quartz-muscovite schist (pelite) and quartz-chlorite-phlogopite marble (meta-dolostone), and containing a mylonitized tectonic breccia; up to 100 m. Flowerdale schist: quartz-biotite-plagioclase schist (semipelite) with narrow amphibole-bearing bands; 500-600 m, but wedging out to NW. Aundrary basite: amphibolite sheet, typically uniform in composition, but varying from highly schistose to unfoliated, enclosing a 30 m wide elongate lens of mylonitized siliceous schist; 500-750 m. Creag Bhan belt: narrow, intensely deformed, brittle-ductile shear zone containing mylonitized and brecciated granodiorite gneiss (Buainichean gneiss) and enclosing a narrow (up to 20 m) band of quartz-biotite-garnet-schist (pelite), a 20 m wide unit consisting of two thin marble (meta-limestone) bands with calc-silicate schist and granitic pegmatite, and several thin schistose amphibolites; up to 150 m. These units of the LMG are bounded on both sides by highly deformed Archaean granodiorite gneisses enclosing numerous amphibolite sheets (deformed Scourie dykes): to the SW, the Shieldaig gneiss, and to the NE, the Buainichean gneiss. The Cloiche belt is separated from the units to the NE by a wedgeshaped outcrop of heterogeneous acid to basic orthogneisses (the lalltaig gneiss) enclosed by shear zones. See Fig. 4.1.

30

CHAPTER 4

Fig. 4.1. Schematic cross-section of the Loch Maree Group outcrop showing the principal rock units (see Table 4.1 for details).

Fig. 4.2. Photomicrographs of LMG metasediments: (A) semipelitic quartz-biotite schist, Flowerdale schist, near Flowerdale Mains [821 747]; note planar biotite-chlorite fabric with biotite porphyroblasts; width of field, 6mm (x15); (B) carbonate- and quartz-rich bands in impure meta-dolostone. Flowerdale marble belt [822 743]; width of field, 6mm (x15).

31

THE LOCH MAREE GROUP

50-60% quartz, 10-20% plagioclase and 20-25% biotite and muscovite. Minor and accessory minerals include chlorite, garnet, apatite, tourmaline and opaque ore. The mean grain size of the quartz and feldspar in these semipelites is around 0.05-0.1mm, except in the Charlestown schist immediately adjoining the Ard gneiss, where the grain size is appreciably coarser (above 0.5mm). Although the present fine grain size is clearly the product of metamorphic and deformational changes, there is no evidence of the breakdown of larger sedimentary grains, and the rocks are thought to have been derived from fine-grained siltstones.

Table 4.2. Chemical analyses of semipelites, from Floyd et al. (1987)

A

B

C

D

67.44 0.73 13.19 5.45 0.05 2.05 1.56 3.33 2.56 0.17 1.03

78.61 0.45 9.77 3.28 0.05 1.00 1.16 2.01 1.60 0.14 0.59

68.46 0.71 12.35 5.33 0.05 2.62 2.09 2.58 2.49 0.17 1.47

4.1.3 Geochemistry and origin

LOI

63.39 0.84 13.04 6.45 0.04 4.16 3.12 2.17 2.95 0.19 2.44

Thirteen samples of semipelitic quartz-plagioclase-biotite schist were analysed for major, trace, and rare-earth elements by Floyd et al. (1989), and the following account is based on this work. The samples were obtained from the Flowerdale (5), Charlestown (5), and Kerrysdale (3) units (Table 4.2). Element abundance levels show marked variations, reflecting a range of original sedimentary compositions. Two main groups can be distinguished: the first includes both the Flowerdale and Charlestown samples and the second the Kerrrysdale samples. The Kerrysdale samples can be distinguished by their significantly higher SiO2 values (71-84% compared with 62-70%) and generally lower Fe2O3, MgO, and CaO values (see Table 4.2 and Floyd et al. 1989, table 1), reflecting the quartz-rich nature of the original sediments. There is some variation in most of the major element values within both groups (see Floyd et al. 1989, table 1), due mainly to variations in the relative proportions of quartz, plagioclase, and biotite, and whether or not muscovite is present. The compositions correspond closely to that of greywacke, as suggested originally by Park (1964) and the high average values of CaO, Fe-Mg components and, particularly Na2O, indicate that the original sediments were relatively immature. Large-ion-lithophile (LIL) elements are very variable (e.g. K2O = 0.45-3.38%) although the LIL element ratios are relatively uniform (e.g. K/Rb = c. 230-300, around the crustal average). Total REE values are high (often > 100 ppm) for semipelitic sediments, and exhibit wellfractionated light-to-heavy REE ratios (La/Lu = c. 60-120). The highly variable nature of the transition trace elements, together with Fe and Mg, reflects the variable proportion of phyllosilicates representing the muddy component of the original sediments. As the local Archaean gneisses generally have much lower K2O and higher K/Rb ratios, the sediments were apparently not of local derivation. Although some mobilization of LIL elements could have taken place during the amphibolite-facies metamorphism, the abundances and ratios of LIL elements match those of unmetamorphosed sediments of similar composition derived from acidic to intermediate igneous precursors, and the K/Rb ratio is generally comparable with the standard crustal value (Floyd et al. 1989, fig. 2). In contrast to the amphibolites enclosed within the Ard gneiss unit to the SW (see Section 5.1.2), there are no quartzofeldspathic veins in the metasediments of the three units sampled. It is therefore considered that LIL element mobility during metamorphism has not been a significant factor, and that it is justifiable to characterize the metasediments on the basis of their major and minor LIL elements. The three plots of Figure 4.3 portray the Gairloch semipelites as a mixture of relatively immature clastic sediments akin to greywackes and lithic sandstones. Relatively immobile elements Ti and Ni are used to discriminate between the group of samples, which falls in the magmatogenic greywacke field, following a typical magmatic trend for intermediate and acidic rocks with variable Ti values, from the other group of samples which defines a sedimentary trend for lithic sandstones with uniform Ti values (Fig. 4.4). The semipelite samples also exhibit a variable chondrite-normal-

Ba Cl Cr Cu Ga Nb Ni Pb Rb S Sr V Y Zn Zr

509 325 306 36 13 11 77 37 110 2700 167 149 23 113 200

765 371 146 17 14 12 36 22 92 404 159 81 34 81 267

367 336 47 8 11 9 18 13 72 168 100 56 19 41 188

567 343 185 22 13 11 48 26 94 1232 148 101 26 84 223

5

5

3

13

SiO2 TiO2 A12O3 Fe2O3t MnO MgO CaO Na 2 O K2O P2O5

n

A, Mean of five samples of Flowerdale schist. B, Mean of five samples of Charlestown schist. C, Mean of three samples of Kerrysdale schist. D, Average LMG semipelite (mean of 13 samples). See Floyd et al. (1987) for full dataset. 'Total iron

ized negative Eu anomaly (Fig. 4.5), low Cr/V (c. 1.7-4.1), high Th/ Sc (c. 0.5-1.5) and high La/Sc (c. 1.6-6.3). These features, together with their high total REE contents, are similar to those of the postArchaean argillaceous sediments (PAAS) compiled by Nance & Taylor (1976). High field-strength element plots of Cr against V and Zr/Nb against TiO2 (Fig. 4.6) effectively discriminate between three sedimentary groups: A, the Flowerdale schist samples; B, the Kerrysdale schist samples; and C, a group of garnet-biotite schists closely associated with the amphibolite sheets (Section 4.7). The Charlestown schist samples have affinities with both groups A and B. These chemical distinctions can also be seen in REE patterns normalized against average post-Archaean argillaceous sediment (PAAS) for the three groups (Fig. 4.7). Group A shows a flat pattern with a small central hump centred on a positive Eu anomaly. This pattern is typical of acidic continental crust except for lower total REE abundances. Group B shows two diverging patterns with the Charlestown schist samples showing progressive enrichment towards the heavy REE and the Kerrysdale schist samples progressive depletion, suggesting variable behaviour of heavy REE either at source or during metamorphism. Floyd et al. (1989) suggest that the heavy REE enrichment in the Charlestown schist samples might reflect selective entrapment in garnet, or complexing in carbonate, whereas the depletion found in the Kerrysdale schist samples might indicate their selective removal during weathering. The intermediate to high silica content and PAAS-like composition of the Gairloch semipelites indicate that these clastic sediments are derived from a continental upper-crustal source. From Sm-Nd isotope data, O'Nions et al. (1983) gave two model crustal residence age estimates of 2.5 and 2.2 Ga respectively for the sediment source

32

CHAPTER 4

Fig. 4.3. Geochemical classification of LMG semipelites as predominantly greywackes and lithic sandstones (from Floyd et al. 1987).

Fig. 4.4. TiO2-Ni plot of LMG semipelites showing one group (metagreywackes) following a typical magmatogenic greywacke trend, and the other (lithic meta-sandstones) following a sandstone trend (from Floyd et al. 1987).

material, obtained on Flowerdale schist samples. These data were interpreted as the result of mixing of late Archaean source material with an older age of c. 2.5 Ga with a component of juvenile crustal material with an age closer to that of the deposition of the sediments (c. 2.0 Ga). However Park et al. (2001) point out that the prominent negative Eu. Sr, and Ba concentrations of the metasediments. which are similar to the average post-Archaean argillaceous sediments (PAAS) of Nance & Taylor (1976) contrast with the neighbouring Lewisian Archaean gneisses, which have either positive or neutral concentrations (see Rollinson & Fowler 1987; Tarney & Jones 1994). The significant heavy rare-earth element (HREE) and Y depletions of the Archaean gneisses are not apparent in the sedimentary patterns, which again are analogous to PAAS. This suggests that the sediments of the LMG were not locally derived. It is possible that highly weathered basalts, enriched in HREE, could have contributed to the high HREE patterns in the sediments, but they could not have caused the prominent negative Nb anomaly nor the highly fractionated REE patterns (the amphibolites have flat patterns). 4.1.4 Zircon age data Further evidence for the sediment source comes from the zircon study of Whitehouse et al. (1997a). Zircons from a Flowerdale schist sample yield a range of ages which fall into two groups: an Archaean group with ages ranging from c. 3.1 to c. 2.5 Ga and an early Palaeoproterozoic group with ages clustering around c. 2.0 and 2.2 Ga. The Archaean zircons may have been derived from local Archaean basement sources. However the early Palaeoproterozoic zircons require a quartzofeldspathic source with ages of c. 2.0 and 2.2 Ga. and although igneous activity in the form of mafic intrusions of c. 2.0 Ga age has been identified in the Lewisian (the later Scourie dyke suite) and of 2.2 Ga age elsewhere in related Palaeoproterozoic provinces in the North Atlantic region, no acid source rocks of that age are yet known. Whitehouse et al. (1997a) conclude that the juvenile component of the sediment was probably derived from a contemporaneous subduction-related volcanic arc which subsequently became displaced during the Laxfordian collision and is now concealed or has been removed by erosion (see Section 7.4.1).

4.2 Other metasediment types

Fig. 4.5. Rock/chondrite-normalized rare-earth element plot of 12 samples of LMG semipelite, from Park et al. (2001).

Narrow bands of carbonate (meta-limestone and meta-dolostone), striped magnetite-quartzite (banded iron formation — BIF), schistose graphite-bearing pelites and other metasediments different from the normal semipelites. occur at numerous localities through-

THE LOCH MAREE GROUP

33

Fig. 4.6. Cr v. V and Zr/Nb v. TiO2 discrimination plots showing separation of LMG semipelites and garnet schists into three chemical groups: A, Flowerdale schists (meta-greywackes); B, Kerrysdale schists (lithic sandstones) and C, garnet-biotite schists, associated with amphibolites. Charlestown schists plot in both A and B groups. From Floyd et al. (1989).

the Kerrysdale basite. Discontinuous bands of carbonate (metadolostone), BIF and, locally, graphitic pelite occur within quartzchlorite schists, which make up the bulk of the outcrop of this belt. Bands of carbonate and BIF also occur within the Kerrysdale amphibolite, and within the Mill na Claise amphibolite, in the hinge zone and on the SW limb of the Mill na Claise fold, south of the River Kerry. A graphitic pelite band occurs intermittently for nearly 2km in the centre of the Kerrysdale schist unit.

4.3 Carbonate (marble) 4.3.1 Distribution and petrography

Fig. 4.7. PAAS-normalized REE plots (see text) for the three chemical groups A, B and C of the LMG semipelites and garnet schists (see Fig. 4.6). From Floyd et al. (1989).

out the district, either within, or at the margins of, the amphibolite sheets (see map). The most prominent outcrop of these rocks forms a narrow lithotectonic unit, up to 150m wide, termed the Flowerdale marble belt, between the Kerrysdale basite and the Flowerdale schist unit, and can be traced for 5 km from Flowerdale southeastwards to Loch Bad an Sgalaig along the NE margin of

Carbonate occurs in narrow bands whose thickness varies widely and rapidly in discontinuous and probably lenticular outcrops. The carbonate occurs in two distinct types: relatively pure, white- to pale grey- weathering, meta-limestone (calcite marble) within acid gneisses along the NE and SW margins of the LMG outcrop; and impure, yellow-weathering, meta-dolostone (dolomitic marble) occurring within the Flowerdale marble belt described above. In a small, shallow, overgrown quarry [855 722] near Am Feurloch in the Creag Bhan belt, meta-limestone forms two bands separated by a narrow hornblende skarn, with a total aggregate width of about 25 m. This occurrence seems to be isolated, although marble also occurs along-strike of this locality, about 2 km to the SE, outside the map boundary. Similar bands of meta-limestone are enclosed within acid Ard gneisses at Coille Airidh na Cloiche [817 720] in a belt which extends in a southeasterly direction to near Loch Druim na Fearna [835 701] and in a northwesterly direction to a point north of the Gairloch-Shieldaig road, where the carbonate is exposed in a small abandoned quarry [811 724] east of Shieldaig Lodge. The outcrop at the quarry is about 9 m wide; much of it is pure calcite, but some parts contain tremolite, quartz, pale green mica and colourless chlorite. Diopside also occurs in some outcrops. The meta-dolostone, which weathers to a distinctive yellow colour, occurs generally as narrower bands within the quartzchlorite schists of the Flowerdale marble belt. A conveniently accessible locality is situated immediately SW of the BIF locality in Flowerdale [818 748] (see Section 4.4.1). This variety consists of alternating bands of relatively pure dolomite marble, mixed with

CHAPTER 4

34

Table 4.3. Chemical analyses of LMG meta-limestones and meta-dolostones from Gairloch; after Rock (1987). Limestones sd SiO2 Al2O3 FeOt MgO CaO Na 2 O K2O TiO2 P2O5 MnO F S

V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Mo Ag Sb Ba La Ce Pb U n t

Dolostones sd

14.8 1.8 3.7 6.1 40.0 0.16 0.15 0.15 0.06 0.09 0.10 0.10

7.4 1.7 2.6 3.1 6.4 0.19 0.24 0.09 0.03 0.04 0.05 0.06

4.5 0.36 1.6 17.2 31.8 0.03 0.05 0.04 0.04 0.25 0.04 0.04

5.1 0.38 1.6 2.9 3.1 0.06 0.09 0.02 0.01 0.19 0.02 0.02

28 12 9 13 25 23 4 4 305 5 21 1 3 1 102 9 62 22 3

13 16 7 10 37 14 1 6 456 3 28 1 1 1 155 18 127 44 4

6 5 3 4 8 14 5

16 11 4 6 13 15 1 3 145 2 5 1 1 1 2721 12 9 16

9

7

212 2 4 1 1 2 898 4 9 10 <1 17

Total iron

small amounts of quartz, and bands rich in green chlorite, with minor phlogopite (Fig. 4.2B). Neither tremolite nor diopside has been found in these occurrences.

4.3.2 Geochemistry A general study of the geochemistry of Lewisian 'marbles' was undertaken by Rock (1987) who included 26 analyses of carbonates and two of associated skarn rocks from the LMG. The analysed samples comprise 17 meta-dolostones, 6 meta-limestones and two calc-silicate rocks (Fig. 4.8). The meta-limestones show a greater spread of silica contents. Mean compositions for the analysed samples are given in Table 4.3. Most elements lie within the normal global range of carbonates (cf. Wedepohl 1978) and are compared with other Lewisian carbonates, Scottish Dalradian carbonates and a worldwide Precambrian carbonate data set by Rock (1987). Unusually high Ba was recorded in one meta-dolostone, and one of the meta-limestones showed particularly high Co, Sr, La, Ce, and Cu. No significant variation in chemistry was found between the LMG carbonates and other Lewisian carbonates, and differences between the meta-limestones and meta-dolostones of Gairloch were considerably greater than those between the Gairloch samples and other Lewisian carbonates. However the Lewisian rocks show generally low Sr and high Mg/Ca compared to more recent carbonates. The greater degree of dolomitization accords with the general increase of dolostone compared with limestone with increasing age noted by Veizer (1978). Within the Gairloch samples, significant differences were noted

Fig. 4.8. MgO v. SiO2 plot of LMG carbonates from Gairloch illustrating grouping into pure and impure limestones, and pure and impure dolostones and skarns; after Rock (1987).

by Rock (1987) between the carbonates of the Flowerdale belt and those of the Cloiche and Creag Bhan belts, reflecting the petrographic differences noted above. The Flowerdale samples show higher metallic trace element contents, presumably related to mineralization (see Section 4.10) and also higher contents of Rb. Sr, Y, Zr, and Ba.

4.4 Banded-iron-formation

4.4.1 Distribution and petrography Outcrops of this distinctive striped magnetite quartzite, known in the Precambrian worldwide as 'banded iron formation' (BIF), occur in narrow impersistent zones varying from a few centimetres to about 20m in width, and consisting of alternating magnetiteand quartz-rich stripes. Individual black or dark brown magnetiterich stripes average about 1 mm in thickness but vary from about 0.25mm to about 10cm. They alternate with pale stripes of quartzrich material with a similar thickness range, containing variable amounts of small grunerite needles and small quantities of calcite. In some cases, the magnetite and quartz are intimately mixed and grunerite is absent; in others, the whole zone is composed of quartzite with only thin folia of magnetite. The quartz typically shows signs of deformation, with irregular or sutured margins and strain bands. One of the most accessible outcrops of the BIF forms a low ridge that runs along the SW side of a field on the south side of the Flowerdale valley at [818 748]. Associated with these striped rocks, and in places occurring along-strike from them, are bands or lensoid masses of coarse garnet-grunerite schist, containing minor quantities of quartz, biotite, and magnetite. Both garnet and grunerite crystals are much larger than is typical of the semipelitic metasediments; sheaves of grunerite are typically around 5mm long and the garnets several millimetres across. The amphibole sheaves are randomly oriented and the rock does not possess a marked foliation, although a crude compositional striping is usually present. The garnets are manganese-bearing spessartites (Tilley 1938: Al-Ameen 1979).

4.4.2 Geochemistry The BIF of the Gairloch district was the subject of detailed studies by Al-Ameen (1979) and Williams (1986). Al-Ameen carried out both whole-rock and mineral analyses on a representative set of 16

THE LOCH MAREE GROUP

samples, including both silicate and oxide-facies rocks, for both major and trace elements. Williams performed only mineral analyses on representative samples of grunerite, hornblende, chlorite (ripidolite), biotite, and garnet. Al-Ameen's analyses show that SiO2 is the main constituent, varying from 46.2% to 64.1% in the silicate facies, and from 42.5% to 64.1% in the oxide facies. TiO2 contents are low with the exception of one sample with 2.0%. A12O3 content in the silicate facies is dependent on the amount of garnet but is generally low in the oxide facies. FeO occurs in garnet and grunerite and varies from 19.4% to 31.4% in the silicate facies. In the oxide facies, FeO varies in the range 14.6-27.6% and Fe2O3 in the range 14.9-29.9%, except for one sample with a very low amount of magnetite. MnO is a major constituent of the silicate facies, occurring in garnet, and varies from 4.1% to 13.3% except for two samples with very little garnet. CaO is present in small quantities, 0.2-3.8%, mainly in garnet, although trace quantities of carbonate are occasionally present. MgO varies from 1.8% to 8.3% in the silicate facies, concentrated in grunerite. Only trace amounts are present in the oxide facies. Na2O and K2O are present only in trace quantities. The principal trace elements in the silicate facies rocks are Cr, Ni, V, Y, and Zr, which are held in garnet and grunerite. In the oxide facies, determined trace element values are very low. Cr, Ni, and V replace iron and increase with increasing iron content. Y substitutes for Mn and reflects garnet concentration. 4,4.3 Origin The layering in the oxide facies of the BIF is considered to be original, although clearly modified by deformation. Williams (1986) notes that studies of iron-formations at various grades of metamorphism and intensities of deformation, in many different parts of the world, have shown that the compositional variation is characteristic of these rocks. It is present at the lowest grades of metamorphism, and survives to the highest grades essentially unchanged. The coarse layering (cm scale), fine lamination (mm scale), and gradation within the layers observed in the Gairloch BIF were all considered by Williams (1986) to be original depositional features, as were the minor variations in concentration of amphibole aggregates and other mineralogical variations. Some of the textural relationships of the metamorphic phases were thought to reflect the distributions of precursor minerals in the protoliths; for example the structures exhibited by the grunerite aggregates suggest derivation from ooliths or granules as found in certain types of iron-rich sediment. Williams considered that the quartz-magnetite assemblages probably recrystallized from primary chert-iron oxide mixtures, and that, as iron-formations metamorphosed at low to medium grade normally preserve pre-metamorphic oxidation states, according to Klein (1973), the protoliths of the Gairloch BIF probably contained magnetite, itself a diagenetic product of sedimentary hydroxides and sesquioxides such as goethite (cf. Mel'nik 1982). The grunerite was thought to form from iron silicates such as greenalite by dehydration and reaction with silica, or alternatively from decarbonation of iron-rich carbonates. The presence of Al, incorporated in minerals such as garnet, biotite, and chlorite, indicates that the protoliths must have contained aluminosilicate phases, probably chamosite. However the biotite, which is the only mineral carrying significant amounts of K and Ti, may be of clastic origin. In summary, Williams concluded that the unmetamorphosed precursors of the BIF consisted of stratified mixtures of minerals derived from chemical precipitates including silica (as chert), iron oxides, chamosite-greenalite silicates and carbonates with probably small amounts of clastic grains that may have included additional silica as quartz. The massive garnet-grunerite rocks imply local concentrations of chamosite-greenalite similar to those described within certain Archaean and Proterozoic BIF by various authors (cf. Williams 1986). Many of these examples are associated with

35

stratiform sulphide deposits, as is the case in Gairloch. Oxidebearing iron formation has been considered to indicate a shallowwater origin (eg. Goodwin 1973; Kimberley 1978). This view is supported by the indications of possible cross-lamination and original oolitic character in the Gairloch rocks which were considered by Williams to point to a shallow-water environment in which chemically precipitated material was reworked by current or wave action. 4.5 Graphitic pelite Schistose graphitic pelite forms a single broad band in the centre of the Kerrysdale schist unit, and also occurs as narrow bands associated with BIF at several localities within the Flowerdale marble belt, within the Charlestown schist unit near Kerry Falls, at [837 719] and in the inner sedimentary band within the Mill na Claise amphibolite, e.g. at [830 718] (see map). The Kerrysdale outcrop provides the best examples. This band is up to 10m wide and consists of brown-weathering schists, locally stained rust- or yellow-coloured, but dark grey on fresh surfaces. The rock is a quartz-muscovite schist with variable amounts of graphite, present either as scattered fine 'dust' between the quartz grains, or concentrated as discrete scales parallel to the micaceous foliation. In the latter case, the graphite can be identified in the field. Good exposures of this variety can be examined in the bed of the small stream that crosses the track, 400 m north of Kerrysdale Farm, at [8225 7370]. In addition to quartz, muscovite, and graphite, some samples contain biotite and/or chlorite, and small amounts of pyrite. Quartz grains are typically highly strained. One sample (Table 4.4, column A) was analysed by Al-Ameen (1979) from a locality in Flowerdale where the graphitic pelite occurs alongside BIF. The sample contains c. 50% graphite, together with roughly equal proportions of quartz and muscovite. The significant amounts of MgO (2.8%) and CaO (also 2.8%) suggest the presence of chlorite or carbonate in addition to the minerals observed. Small amounts of iron are probably represented by the minor pyrite. The presence of 3.1% Na2O is puzzling and probably indicates the presence of unrecorded plagioclase (mistakenly identified as quartz).

4.6 Quartz-chlorite schist 4.6.1 Distribution and petrography Quartz-chlorite schist makes up the main component of the Flowerdale marble belt, and also occurs in narrow bands within certain amphibolite sheets (see map). The greenish-grey chlorite schists of the Flowerdale marble belt are in places difficult to distinguish from the hornblende schists of the adjoining Kerrysdale basite, but are softer and more distinctly schistose, commonly with a banded structure imparted by variations in the proportions of calcite and quartz compared to chlorite. Quartz and chlorite are the two dominant constituents, together with variable amounts of calcite. Some samples contain hornblende as scattered porphyroblasts and others contain a pale green mica (possibly fuchsite) in addition to chlorite. Plagioclase may be present as a minor constituent. Apatite, tourmaline, and opaque ore occur as accessories. The proportion of quartz varies considerably. Some varieties are almost pure quartzite, with minor quantities of calcite and small chlorite needles, whereas others are highly schistose and dominated by chlorite. With the exception of the hornblende porphyroblasts, which are up to 1 mm or more in length, where present, these rocks are similar in grain size to the neighbouring semipelites. In the northeastern part of the outcrop of the Flowerdale marble belt, at [825 736], a narrow shear zone, about 20m wide, contains both brittle cataclastic rocks and more ductile mylonites. The mylonitic rocks include a highly deformed fragmental rock

36

CHAPTER 4

Table 4.4. Chemical analyses of LMG metasediments (minor bands) from Gairloch

A SiO2 TiO2 A12O3 Fe2O3 FeO MnO MgO CaO Na 2 0 K2O LOI S P2O5

56.9 1.00 16.6 1.3 0.9 0 2.8 2.8 3.1 2.0 11.44L

C

D

E

58.28 0.55 11.55 1.57 13.44 0.23 8.12 1.75 1.38 <0.12 1.185

55.77 0.68 13.90 0.95 9.95 0.16 6.18 5.69 1.41 0.25 -

59.06 1.68 12.82 4.22 9.38 0.28 3.88 2.72 1.32 2.07 -

48.64 0.29 8.22 t

0.85 3.93 2.44 0.73 0.67 -

58.6 0.67 10.27 3.05 2.75 1.16 3.32 1.46 1.36 -

78 121 <5 30 186 225 -

0.08 78 128 6 192 11 5 52 144 15 26 -

_ 607 241 114 12 79 34 67 134 268 20 139 121 17.5 36.5

0.52 80 106 159 203 159 62 28 509 461 152 -

0.36 27 55 319 329 44 490 -

4

1

6

9

2

_

Ba Co Cr Cu Nb Ni Pb Rb Sr V Y Zn Zr La Ce n

B

1

26.7 -

F

A, graphitic pelite; B, C, quartz-chlorite schist; D, biotite-garnet schist; E, Troodos mudstone; F, off-mound ooze. From: (A, B) Jones et al. (1987); (C) Al Ameen (1979); (D) Floyd et al. (1989): (E) Robertson (1978); (F) Migdasov et al. (1983); see text for details. LOI, Loss on ignition; S. sulphur t Total iron

consisting of flattened and elongate fragments of quartzite, up to several centimetres across, together with less abundant fragments of calcareous and chloritic schist, in a matrix of calcareous quartzchlorite schist. This rock extends for a few hundred metres in a southeasterly direction, but is not seen elsewhere in the Gairloch district. The brecciation is considered to predate the ductile shearing, and the rock is thought to represent a pre-metamorphic tectonic breccia marking the site of an early decollement plane that separated two tectonic slices during the accretion of the LMG (Park et al. 2001). The breccia was subsequently subjected to very high strains in the early Laxfordian shearing. Quartz-chlorite schists form narrow zones within the Aundrary basite unit, near the summit of Sidhean Mor, in square [83 74], and within the Kerrysdale basite, running SE from a point east of Auchtercairn, at [807 768]. Both consist of quartz-rich chlorite schists, and contain bands of finely granular quartz with minor quantities of biotite and epidote; titanite, apatite, and tourmaline occur as accessories. 4.6.2 Geochemistry and origin Six analyses of major and selected trace elements were carried out by Jones et al. (1987) from a chlorite schist band within the Kerrysdale basite unit, and one by Al-Ameen (1979) from a chlorite schist from the Flowerdale marble belt (Table 4.4, columns B, C). The values showed wide variations, notably in SiO2 (42.2-58.3%), Fe2O3 (tot.) (12.0-16.0%), MgO (6.2-22.0%) and CaO (1.8-7.0%).

Alkalis are low. Such a range is unsurprising, given the variations in mineral content encountered in these schists, for example the amount of carbonate present. The Flowerdale chlorite schist sample shows more affinities with the metavolcanic amphibolites with which it is associated than the Kerrysdale samples. Compared with the amphibolites, the Flowerdale sample has increased Si and reduced Ca and Na. These schists occur stratigraphically above the metabasalts, and probably represent volcaniclastic sediments, mixed with additional siliciclastic material (either from chert or of clastic origin) and from which some Ca and Na have been leached out. Their association with the chemical sediments (BIF. marble, etc.) is discussed below. The Kerrysdale chlorite schist samples also have higher SiO2 values than the amphibolites, and generally higher Mg, but lower Ca. Na. and K. Of the trace elements, Cu, Zn, and Zr are higher than in the amphibolites, and Sr. Cr, and Rb lower. Jones et al. (1987) considered that this chlorite schist, which directly underlies the ore horizon, probably represents basalt, that has been hydrothermally altered in situ. 4.7 Garnet-biotite schist 4.7.1 Distribution and petrography Although the semipelites of the Charlestown and Kerrysdale schists locally carry small garnets, these semipelitic units are generally not garnetiferous. However, narrow, brown-weathering bands of distinctive garnet-rich schist occur within the Creag Bhan belt, NE of the Aundrary basite, within the amphibolites of the Cloiche belt at Coille Airigh na Cloiche. and within the inner metasedimentary band of the Mill na Claise basite. in the fold hinge (see map). The Creag Bhan band can be traced intermittently for about 2.5km and good exposures occur at [854 722], north of Am Feurloch and just west of the marble outcrop. This rock contains elongate garnet porphyroblasts around 1 cm in length in a highly schistose matrix of quartz and biotite. The rock also contains minor plagioclase, muscovite. epidote. and chlorite, and accessory tourmaline and apatite. The composition of these garnet schists is unlike that of the Flowerdale or Kerrysdale semipelites in several respects (cf. Section 4.1.2). and they are appreciably coarsergrained. The garnets are deformed into augen. with 'tails' of hornblende and biotite. The bands of garnet-biotite schist in the Cloiche belt contain garnet porphyroblasts. about 5 mm in diameter, wrapped by biotite and hornblende folia. Quartz is abundant and plagioclase is also present in relatively small amounts. Good exposures occur on the hill immediately south of Coille Airigh na Cloiche. at [816 717]. 4.7.2 Geochemistry and origin Six samples of the garnet-biotite schist, which occur as bands within or at the margins of several of the amphibolite sheets, were analysed by Floyd et al. (1989) (Table 4.4. column D). As in the case of the quartz-chlorite schists, the close association of these rocks with the metavolcanic units suggests a genetic relationship. Despite the abundance of garnet, these schists do not have a pelitic composition, having low alumina (mean A12O3 = 12.8%) and alkalis (mean Na 2 O + K 2 O = 2.1%). However, in comparison with the average metabasalt composition, they are considerably richer in SiO2 (c. 59%) and K2O and poorer in MgO and CaO. The trace element values are equally anomalous. Most are similar to those of the semipelites (Ba, Nb. Pb. Rb, and Zr) or are similar to both the amphibolites and the semipelites. V has a similar value to the amphibolites and Cu is even higher. The rare-earth elements show marked light REE depletion relative to PAAS (see Fig. 4.7. group C). Floyd et al. (1989) suggested the possibility that these rocks represent a mixture of basaltic debris or tuff and 'normal' greywacke-type sediment. Thus the rather unusual REE pattern would reflect the influence of amphibolite material, as the

THE LOCH MAREE GROUP

37

Fig. 4.9. (A) Intensely foliated, schistose amphibolite showing narrow segregation stripes of quartzo-feldspathic material, within Ard gneiss outcrop, Druim Ruadh [821 725]. (B) Prominent quartzo-feldspathic stripes and bands in amphibolite from within Ard gneiss outcrop, Loch Kerry [812 735]. Ruler is 15cm

horizontal chondrite-normalized REE patterns exhibited by the associated metavolcanic amphibolites (Johnson et al. 1987) (Fig. 4.14A) would produce a depleted light REE pattern against PAAS, as seen in Figure 4.7. However a straightforward mixture of these two end-members (i.e. basalt and greywacke) would not explain that although some elements have intermediate values (e.g. Si, Al) others have values much closer to those of the end-members themselves (see Table 4.4). For example, Fe, Mn, and Ti have 'basic' values while MgO, CaO, and K2O have 'semipelite' values.

In order to reach the required composition, the normal semipelite sediment would require substantial enrichment in the former (especially Fe), but no enrichment in either Mg or Ca, and some process other than admixture of mafic detritus is required. A more likely explanation is that the basaltic protolith of the amphibolite immediately adjacent to these schists was affected by hydrothermal activity which produced material enriched in Fe, Mn, Ti, and V, but depleted in Na and Ca. This material was then mixed with the 'normal' marine sediment.

38

CHAPTER 4

Fig. 4.10. (A) Photomicrograph of fine-grained schistose amphibolite from Aundrary amphibolite, north shore of Loch Bad an Sgalaig [852 719]: width of field. 9mm (x7.5). (B) Photomicrograph of coarse-grained, weakly foliated amphibolite from the Ard gneiss outcrop. Loch Kerry [810 736]: width of field, 10mm (x7.5).

Robertson (1978) describes iron-rich mudstones associated with basalt lavas in the Troodos Massif in Cyprus. These mudstones are considered to have originated as chemical precipitates from hydrothermal solutions generated by leaching of basalt by heated circulating seawater. The mudstones are enriched in Fe, Si, K, Mn, and P, but depleted in Al, Ba, Mg, Ca, Cr, Sr, and Ni. Nine analyses are provided by Robertson, and average values are shown in Table 4.4, column E. Most elements show a wide range of values which encompass the garnet-schist values with the exception of K, Ti, and Ba, which are higher in the garnet-biotite schists, and Ni and Zn, which are lower. Comparisons can also be made with the composition of pelagic sediments influenced by hydrothermal activity in the Galapagos Mounds area of the Eastern Pacific (Migdasov et al. 1983). Pelagic oozes formed here at some distance from the hydrothermal mounds also show a close chemical similarity with the Gairloch garnet-biotite schists (Table 4.4, column F) except for much lower Fe. These Cretaceous and recent examples of sediments derived from hydrothermal alteration of basalts suggest that this is an appropriate model for the origin of the Gairloch garnet-biotite schists. The model is strengthened by the presence of the hydrothermal stratiform ore horizon and the chemical deposits of the Flowerdale marble belt at stratigraphically higher levels.

4.8 Origin of the BIF-carbonate-graphitic pelite assemblage The origin of the unusual metasedimentary rocks closely associated with the amphibolites is best considered separately from the apparently straightforward origin of the bulk of the (semipelitic) sediments. Current views on the origin of BIF favour a shallowmarine, biogenic origin, which would also explain the presence of the associated graphitic schists. The carbonates may also have had a biogenic origin, although there is no local supporting evidence for that view. These largely chemical sediments appear to have formed on a substrate of submarine basalt lavas, together with other sediments similar to the units of metagreywacke such as the Flowerdale schists. An important component of this assemblage is the quartz-chlorite schists; these are interpreted as altered basaltic volcaniclastic material to which variable amounts of silica have been added, possibly due to admixture of chert, and from which Ca has been redistributed into separate layers.

The stratiform sulphide deposits described in Section 4.10 yield further supporting evidence for this model. As the main ore zone has been affected by strong ductile deformation, the orebody is considered to be pretectonic in origin. Its close spatial association with the mafic igneous rocks, together with the BIF and other chemical sediments, suggests an exhalative origin. The Mg-rich chlorite schists in the immediate footwall of the ore deposit are thought by Jones et al. (1987) to represent hydrothermal alteration of the basalt protolith. An alternative possibility, that the chlorite schists are the product of retrogressive alteration of the amphibolites during shearing, is deemed unlikely.

4.9 Amphibolites 4.9.1 Distribution and petrography The amphibolites of the LMG occur as distinct, NW-trending, steeply dipping sheets separated by metasediments (see map). Three main sheets are distinguished (Table 4.1): the Aundrary, Kerrysdale and Mill na Claise basites, each of which encloses metasedimentary bands; in addition, there are a number of thin amphibolite sheets within the Flowerdale schist, Charlestown schist, Ard gneiss, and Cloiche units. The amphibolites typically weather to a dark grey colour, and form well-exposed, upstanding, rocky ridges in contrast to the adjoining smoother and less well-exposed ground occupied by the metasediments. There is considerable variation in the development of foliation and lineation, and in grain size throughout the district. The amphibolites range from strongly-foliated, fine-grained hornblende schists (Fig. 4.9A) to coarse-grained, poorly foliated or unfoliated varieties. Considerable variation is seen even within individual sheets. Foliation is typically expressed by the development of alternating stripes or elongate lenses of hornblende-rich and feldspar-rich material (Fig. 4.10A). This striping is considered to be a product of metamorphic segregation aided by the intense deformation rather than of original compositional differences. A schistose fabric caused by the preferred orientation of hornblende prisms normally accompanies the compositional striping but is by no means universally present, some varieties of amphibolite being marked by randomly arranged hornblende crystals. In the schistose varieties, the hornblendes are either aligned randomly in the plane of the foliation, producing an S-fabric. or aligned with their long

39

THE LOCH MAREE GROUP Table 4.5. Chemical analyses (average) of LMG amphibolites

SiO2 TiO2 A12O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 Ba Cr Cu Ga Nb Ni

Pb Rb Sr Th V Y Zn Zr La Ce n

AX

B6

B7

BX

49.79 1.18 14.24 4.93 8.3 0.21 7.23 9.0 2.72 0.58 0.10

49.45 1.14 14.20 2.39 9.97 0.17 6.93 9.78 2.43 0.30 0.10

52.04 1.55 15.64 1.72 9.39 0.13 5.32 8.67 2.66 0.80 0.21

50.46 1.62 15.75 1.22 9.89 0.16 4.88 7.01 3.83 1.66 0.23

51.25 1.59 15.70 1.47 9.64 0.15 5.10 7.84 3.25 1.23 0.22

74 121 87 18 6 90 0.5 5 152 2 332 25 115 82 8 24

116 199 55 19 5 106 0.5 19 175 2 322 23 108 76 3 19

75 159 92 18 5 96 0.5 9 149 1 315 24 103 73 4 19

198 180 57 20 7 72 1 20 289 4 295 29 117 122 13 35

458 79 81 19 5 37 2 49 581 1 403 30 107 130 16 36

328 130 69 20 6 25 1.5 35 435 2.5 318 30 121 126 15 36

17

17

17

18

Al

A2

A3

A4

A5

49.63 1.02 14.64 1.69 10.56 0.18 7.28 10.13 2.35 0.18 0.08

48.53 1.27 13.50 1.65 10.59 0.16 6.27 9.54 2.48 0.19 0.13

49.88 0.95 14.57 1.52 10.23 0.15 7.27 10.27 2.44 0.24 0.09

49.44 1.19 14.03 2.15 10.19 0.16 6.59 9.94 2.17 0.30 0.11

54 174 95 17 4 100 0.6 3 134 1 300 21 98 65 4 17

50 142 93 18 4 84 0.5 2 137 1 338 26 100 84 4 16

83 160 78 18 5 98 0.6 5 145 1 283 23 95 60 2 19

44

91

22

Al, Aundrary amphibolite; A2, Kerrysdale amphibolite (NE branch); A3, Kerrysdale amphibolite (SE branch); A4, Mill na Claise amphibolite; A5, Cloiche amphibolite; AX, mean of A1-A5; B6, thin amphibolite in Charlestown schist; B7, thin amphibolite in Flowerdale schist; from Johnson et al. (1987); BX, mean of Bl, B2. For standard deviations, see Johnson et al. (1987).

axes parallel, forming an LS-fabric. Striping is much more pronounced in those amphibolites within the Ard gneiss unit in the southwestern part of the LMG belt, where individual stripes reach 1 cm or more in width (Fig. 4.9B). Hornblende is typically the most abundant mineral, although varying widely in amount from about 35% to around 80% of the rock. In thin section, it is characteristically green to bluish-green in colour, but is paler and less markedly pleochroic in the highly schistose varieties of the Aundrary and Kerrysdale sheets than is the case in the coarser amphibolites to the SW. Plagioclase is either calcic oligoclase or andesine, and varies from about 10% to over 50% of the rock. It typically forms polygonal grain aggregates along with quartz, which makes up less than 10% of the rock. Epidote is present in most samples, normally only in trace proportions in the hornblende schists, but is more abundant in the coarser amphibolites associated with the Ard gneiss. Garnet occurs in certain amphibolites in the more southwesterly outcrops, forming porphyroblasts of between 0.5 and 2mm in diameter, but is not uniformly present in any individual sheet. Typically it is partly replaced by aggregates of plagioclase, hornblende, magnetite and epidote. Biotite is present in all the main amphibolite sheets but not in every sample, and varies from trace quantities to over 10% of the rock. Chlorite appears in minor amounts as a breakdown product of hornblende. Calcite is present in some samples in scattered single grains and also in veins, and is considered to be a retrogressive product. Accessory minerals include titanite, apatite, and opaque ore, the latter comprising mainly magnetite, in places partially altered to haematite. Ilmenite and, more rarely, pyrite, are also found. The mean grain size, as expressed in the size of the hornblende crystals, is extremely variable, from less than 0.1 mm across in the

well-foliated hornblende schists of the Aundrary and Kerrysdale amphibolites (Fig. 4.10A) to around 1mm in the coarser-grained southwestern sheets (Fig. 4.10B). The coarsening of the grain size in the more southwesterly sheets is accompanied by a marked increase in the scale and extent of the feldspathic striping. Plagioclase also shows an increase in grain size in the southwestern sheets but is even smaller in average size in the northeastern sheets. The most conspicuous segregation effects are seen in the amphibolite which runs from Loch Kerry, at [811 737] southeastwards along the SW side of the Mill na Claise fold to Dubh Loch (see map). This body contains prominent stripes, bands and lenses of both feldspathic and ultramafic material, and was described in detail by Bowes & Park (1966). The larger ultramafic lenses, believed by Bowes & Park to be products of metamorphic segregation, are now considered to represent original ultramafic igneous material - the largest is 380m long and over 30 m wide - and there is doubt as to whether the Loch Kerry body belongs to the LMG, or whether it belongs to the 'early basic' suite of the Scourian complex (see Section 2.1.3), which it resembles superficially. As noted earlier, the Aundrary amphibolite sheet contains some of the most finely-foliated hornblende schist, but it also contains coarse-grained amphibolite (with a mean grain size around 1 mm) in the form of a discrete band, up to 200 m in width, in the central part of the sheet. This band can be traced for about 1 km from Meall Aundrary southeastwards to the Gairloch-Kinlochewe road (see map). Further outcrops of this rock type occur north of the Flowerdale fault between An Groban and Meall Fuaraidh. The coarse amphibolite is completely unfoliated in places, but elsewhere shows a crude foliation, and grades into a well-foliated variety at its margins. Some of the coarser-grained amphibolites display possible relict ophitic or sub-ophitic textures, with lath-shaped feldspar

40

CHAPTER 4

Fig. 4.11. Ni v. TiO2 and Cr v. TiO2 plots after Leake (1964) of LMG amphibolites showing a crude igneous trend and scatter outside the para-amphibolite (pelite) field; from Johnson et al. (1987).

aggregates representing originally much larger feldspar crystals enclosed within mafic material now completely recrystallized to hornblende aggregates. However, petrographic evidence which might point to an igneous origin for these amphibolites is generally obscured by the metamorphic and deformational effects. Several bands of unusual composition are found within both the Kerrysdale and Mill na Claise amphibolites. These bands are much coarser-grained than the adjoining rock and carry either actinolite or tremolite in place of hornblende. The felsic aggregates contain abundant epidote or zoisite and appear to be pseudomorphs after original large feldspar crystals of the order of 5 mm to 1 cm in size. One such band forms an elongate outcrop in Kerrysdale at [818 743]. On the SW side of this outcrop is a small lenticular band consisting of altered plagioclase, quartz, biotite, and chlorite, in which large relict grains now represented mostly by granular feldspar-quartz aggregates are separated by more finely granular material. This is the only recorded example of this type of rock within the amphibolites; it is of granitic composition and is thought to represent a volcaniclastic fragment of either acid gneiss or granite. The accompanying actinolite or tremolite schists may represent a feldspathic tuff horizon. A distinct type of amphibolite occurs as narrow bands within the semipelites of the Flowerdale schist and Charlestown schist units. These amphibolites are typically schistose, and contain varying amounts of biotite. They are appreciably coarser in grain than the adjoining semipelites, mainly due to the larger size of the hornblende crystals. Some varieties contain actinolite or tremolite instead of hornblende and appreciable amounts of quartz. Calcite, epidote, and chlorite may be present in addition, and apatite and opaque ore occur as accessories. Two main bands of these amphibole-bearing schists within the Flowerdale schist unit can be traced for about 2 km northwestwards from Kerrysdale, converging on the slopes of Meall na h'Inginn, above Flowerdale glen, around [825 745] in a zone having the appearance of a complex fold closure.

4.9.2 Geochemistry Johnson et al. (1987) published major and trace-element analyses on 154 samples of amphibolite collected from seven separate sheets: the Aundrary (44 analyses). NE Kerrysdale (19). SW Kerrysdale (22), Mill na Claise (17) and Cloiche ( 1 7 ) bodies, together with two thin sheets from within the Charlestown schist unit (17) and Flowerdale schist unit (18) respectively. Average compositions for each of these bodies are given in Table 4.5. The bodies were divided into two groups, A and B, based on marked geochemical differences in, for example SiO2, A12O3, Ba, Sr, and Zr. Within each body, and between the means of each of the bodies in group A, there is remarkably little variation in either major or trace elements, confirming the homogeneous appearance of the bodies in the field. The large number of analyses from the Aundrary sheet (44) coupled with the low standard deviations (Johnson et al. 1987) show that this body in particular is extremely homogeneous. This uniformity in composition implies an igneous origin, which is confirmed by Ni TiO2 and Cr TiO2 plots (Fig. 4 . 1 1 ) indicating a scatter of points well outside the fields of para-amphibolite. and a weakly defined igneous trend. The generally basaltic nature of the amphibolites suggested by the earlier work of Park (1966) is confirmed by the much larger data set of Johnson et al. (1987) and is indicated especially by the low SiO2 and K 2 O values and high total iron, MgO and CaO. Enhanced K2O, Rb, and Ba concentrations in some samples suggest the possibility of minor local metasomatic alteration. Evidence of limited fractionation is provided by weak Fe and Ti enrichment trends in the AFM diagram (Fig. 4.12A) and in a TiO2 v. Zr P2O5 plot (Fig. 4.12B); the latter also groups the amphibolites in the subalkaline, tholeiitic field. The extent of high-level fractionation within individual bodies was assessed by Johnson et al. (1987) using a Zr/Y-Zr plot. The Aundrary basite. although the thickest of the sheets, possesses the most homogeneous chemistry, with little evidence of high-level fractionation. The other Group A bodies show distinct but limited fractionation trends within the basalt

THE LOCH MAREE GROUP

41

Fig. 4.12. AFM (A) and TiO2 v. Zr/P2O5 (B) plots of LMG amphibolites, showing concentrations within the tholeiitic field and weak iron and Ti enrichment trends; from Johnson et al. (1987).

compositional range. One Group B body also has a well-defined fractionation trend, but the other is relatively homogeneous. However, differences in high-level fractionation, cannot explain the differences between Group A and Group B chemistry. Sixteen rare-earth-element (REE) analyses undertaken by Johnson et al. (1987), 10 from Group A and 6 from Group B, are shown in Figure 4.13. All but one of the Group A samples show flat REE profiles with sample/chondrite values of around 10. The exception is a sample from the Kerrysdale sheet, which shows slight light REE enrichment. Samples from bodies A4 and A5 (see Table 4.5) show slight LREE depletion. The Group B samples show quite different patterns with moderate LREE enrichment and without Eu anomalies, similar to those of the Scourie dykes (Fig. 4.13B) (e.g. see Tarney & Weaver 1987b). This difference is reinforced by a mantle-normalized multi-element plot (Fig. 4.14), which shows flat

Group A profiles with some LILE depletion, whereas the Group B profiles are characterized by LILE enrichment with prominent negative Nb and Ta anomalies.

4.9.3 Origin The geochemistry of the Group A amphibolites leaves little doubt that these rocks are of basaltic igneous origin. Although direct evidence in the form of relict pillows, for example, is not available, the intercalation with chemical and clastic sediments of marine type, together with the evidence of associated hydrothermal activity, are strong indicators of a submarine origin, and suggest that the basalts were erupted in an active ridge setting. Johnson et al (1987) recognized the strength of the chemical arguments for an

42

CHAPTER 4

Fig. 4.13. (A) Chondrite-normalized plot of LMG amphibolites showing distinct Group A and Group B profiles: Group A with flat patterns and Group B with more LREE-enriched patterns. (B) REE patterns of Scourie metadolerite dykes from the Central Region for comparison; note resemblance to Group B amphibolites. A, B from Park el al. (2001).

oceanic origin but concluded that the intercalation of the LMG within continental basement, together with the lack of evidence of a volcanic arc, argued instead for an intraplate setting. However the chemical evidence of an oceanic origin is here considered to be conclusive, and other evidence of a subduction/collision setting for the LMG is discussed in Section 7.4). In their appreciable iron-enrichment (up to 18 wt % Fe2O3) and Ti-enrichment trends, the Group A amphibolites resemble Icelandic basalts (cf. Tarney el al. 1979) and basalts from other plateaus such as the Caribbean/Colombian example (Kerr el al. 1996, 1997a,b; White el al 1999) and Ontong Java (Babbs 1997; Mahoney el al. 1993). Although most Group A samples plot within the mid-ocean ridge basalt (MORB) field on discrimination diagrams (Johnson el al 1987), they are not LREE depleted like N-type MORB, but have flat patterns, without Nb/Ta depletion (Figs 4.13, 4.14). This appears to be a characteristic feature of basalts from ocean plateaus (cf. Kerr el al 1997a,b). The volumetrically subordinate Group B bodies are thin sheets, typically composed of biotite-amphibolite, that occasionally diplay relict igneous textures. Park (1964) suggested a volcaniclastic origin for certain of these sheets, but they are now considered to represent igneous sills. However the origin of the more siliceous amphibolebearing schists which are spatially associated with the biotiteamphibolites is uncertain; some may represent altered sediments, others perhaps more acid igneous fractions.

The emplacement of these Group B bodies may have been much later than the extrusion of the Group A basalts and hence may represent a different tectonic environment. They differ from the Group A bodies in exhibiting a more calc-alkaline trend, and have some superficial geochemical resemblances to Scourie dykes (Figs 4.13, 4.14). However the Scourie dykes were probably emplaced much earlier, and differ in having much lower Sr content. Compared with Group A amphibolites, the Group B bodies have much higher levels of Ba, Sr and P, and much lower V at equivalent Zr values. In addition, they have more fractionated REE patterns and significant Nb-Ta depletion (Johnson el al 1987; Fig. 4.14). Park el al (2001) suggest that the closest modern analogues may be found in primitive island-arc systems such as the Marianas in the west Pacific. 4.10 Stratiform sulphide deposits within the Loch Maree Group Two laterally extensive sulphide horizons occurring within the Kerrysdale amphibolite have been described by Jones el al. (1987). One horizon, which has been traced intermittently for about 6 km parallel to the strike, consists mainly of iron sulphides. The other, a copper-bearing deposit about 4m thick and 1 km long, is a quartzcarbonate schist with pyrite, pyrrhotite, chalcopyrite, sphalerite, and native gold. Both sulphide deposits are considered by Jones el

THE LOCH MAREE GROUP

43

Fig. 4.14. Multi-element mantle-normalized plots: (A) average compositions of Group A and Group B LMG amphibolites with average N-type MORB (Sun & McDonough 1989) for comparison; note prominent Nb anomaly in Group B but none in Group A; (B) Scourie metadolerite dykes for comparison; these show

al. to be of exhalative origin. The Cu-Zn deposit was noted by C. T. Clough (in Peach et al. 1907). The exposed part of this deposit crosses the old Gairloch-Kinlochewe road, now a track, at [838 725]. Jones et al. (1987) describe the ore body as a malachite-stained gossan after pyrite-bearing quartz-carbonate schist, which can be traced along strike for about 200m and has been proved by geophysical methods for a further 500m. The average width is 4m. The ore is fine grained and banded on a millimetre scale, with a variable quartz/carbonate ratio. The carbonate is mainly calcite with subordinate ankerite. Some thin bands contain chlorite, biotite, talc, actinolite, garnet, and tourmaline. Sulphides typically make up 15-20% of the rock and consist chiefly of pyrrhotite and pyrite, with minor chalcopyrite, sphalerite, marcasite, and (rare) galena. Rare native gold and accessory magnetite are also present. Jones et al. (1987) considered that the pyrite and marcasite postdated the Laxfordian ductile deformation but that the other sulphides were pre-tectonic. The ore body is underlain by a 20-30 m thick band of sheared chloritic hornblende schist with quartz lenses and is overlain by 'normal' hornblende schist. Jones et al. (1987) interpret the chloritic schist as the hydrothermally altered footwall to the ore body. The ore horizon is bounded to the NE and SW by thrust faults. A thin (<10cm wide) gossan occurs about 300m along strike to the SW, at Teangadh Buidhe Mhor, also containing significant quantities of Cu, Zn, and Au; however the extent of the mineralization there appeared to be limited.

Several other minor occurrences of sulphide ores are described by Jones et al. (1987). One is at Sidhean Mor, within the Aundrary amphibolite, where a narrow gossan containing pyrrhotite and pyrite can be traced for about 6 km. The gossan is at its thickest at the NW end of the outcrop where it occurs within the mapped unit of siliceous schist about 300m from the SW margin of the amphibolite sheet. Low values of Cu, Zn, and Au were found and the body appears to consist predominantly of iron sulphides. A second occurrence was noted at Druim na Fearna, at [835 702] within the southeastern part of the Cloiche marble belt, where a sulphide-bearing horizon 6-10m in thickness occurs within an amphibolite sheet between the two marble bands. Chalcopyrite, bornite, malachite, and chalcocite occur along with pyrrhotite, pyrite, and magnetite over a strike distance of 220m. Further minor occurrences of chalcopyrite were noted along strike to the NW, but the mineral potential of this belt was considered to be low. The only deposit on which detailed work has been done, and for which chemical analyses are available, is the Kerry Road sulphide deposit (see above). Fifteen partial analyses are provided by Jones et al. (1987). Significant quantities of chalcopyrite and sphalerite are reflected in Cu values of up to 1 % and Zn values of up to 2%. Lead values are low. The rare native gold is rich in silver (20-46%). The pre-Laxfordian age of the mineralization and the association with oxide, sulphide, and silicate deposits (BIF) interpreted as exhalates, suggest that the ore body also has an exhalative origin.

44

CHAPTER 4

4.11 Origin of the Loch Maree Group

4.11.1 Semipelites The intermediate to high silica content and PAAS-like composition of the Gairloch semipelites indicate that these clastic sediments are derived from a continental upper-crustal source (Floyd el al. 1989). The Sm-Nd isotope data of O'Nions et al. (1983), based on semipelite schist samples, gave two model crustal residence age estimates of 2.5 and 2.2 Ga for the sediment source material. These data are interpreted to reflect mixing of late Archaean (c. 2.5 Ga) source material with a component of juvenile crustal material with an age closer to that of the deposition of the sediments (c. 2.0 Ga). More detailed supporting evidence for this sediment source model comes from the zircon study of Whitehouse et al. (1997a). Zircons from a semipelite schist sample (Flowerdale schist) yielded a range of ages that fall into two groups: an Archaean group with ages ranging from c. 2.5 to c. 3.1 Ga and an early Palaeoproterozoic group with ages clustering at around 2.0 and 2.2 Ga. The Archaean zircons are considered to have been derived from Archaean basement sources with similar ages and thus confirm the evidence of a continental source indicated from the geochemical data (see Section 4.1.3). The early Palaeoproterozoic zircons require a quartzofeldspathic source with ages of c. 2.0 and 2.2 Ga, and although mafic magmatism occurred in the Lewisian at 2.0 Ga (the later Scourie dyke suite) and elsewhere in the North Atlantic region at 2.2 Ga, no acid source rocks of these ages are yet known, either locally or regionally. Whitehouse et al. (1997a) concluded that the juvenile component of the sediment was probably derived from a contemporaneous, subduction-related, volcanic arc that is now concealed or has been removed by erosion.

4.11.2 Other metasediments The origin of those metasedimentary rocks closely associated with the amphibolites must be considered separately from that of the semipelitic sediments. Current views on the origin of banded ironformation favour a shallow-marine, biogenic origin, which would also explain the presence of the associated graphitic schists. The carbonates may also have had a biogenic origin. These largely chemical sediments must have formed on a substrate of submarine basalt, on which variable amounts of sediment chemically

indistinguishable from the larger units of metagreywacke were also deposited. The quartz-chlorite schists and garnet-biotite schists are important components of this assemblage. These are thought to represent basaltic volcanic material that has been hydrothermally altered via silica addition (possibly due to admixture of chert) and Ca redistribution. The stratiform sulphide deposits yield further supporting evidence for this model. The fact that the main ore zone has been affected by strong ductile deformation indicates that the orebody is pretectonic in origin and its close spatial association with the mafic igneous rocks, together with the banded-ironformation and other chemical sediments, suggests an exhalative origin.

4.11.3 Amphibolites The chemistry of the Group A amphibolites of the Loch Maree Group (with their flat REE patterns) is consistent with that of an oceanic plateau basalt, whereas that of the Group B amphibolites (with more fractionated REEs, high Sr and Ba. and negative NbTa anomalies) is consistent with primitive island arc basalts or basaltic andesites. The oceanic plateau basalts and arc basalts may be independent in origin or they could be closely associated (White et al. 1999; Petterson et al. 1999). The Fe- and Ti-rich nature of the Group A amphibolites could be explained by the trapping of magma chambers within the overthickened (10-30 km) basaltic crust of a plateau (Kerr et al. 1998). Alternatively, the mantle plume that forms the oceanic plateau is likely to contain a large proportion of recycled basaltic crust that is then available as basaltic partial melts (Cordery et al. 1997: Tarney et al. 1997: Takahashi et al. 1998). In fact Takahashi et al. (1998) have emphasized that where thick lithosphere inhibits adiabatic ascent of the plume, more silicic (basaltic andesite) magmas may result. Hence, most of the petrological and geochemical features in the Loch Maree amphibolites (types A and B) are consistent with an oceanic plateau origin. The LMG thus consists of two quite distinct assemblages: one dominated by metagreywackes formed proximally to a continent and also influenced by a contemporary magmatic arc; the other, consisting of the plateau-type metabasalts and their associated sediments, probably formed in an oceanic setting. Although these two assemblages could have formed in close proximity, it is more likely, given the evidence for an active margin setting, that they have been juxtaposed tectonically.

Chapter 5 Laxfordian Intrusive Rocks Igneous intrusions of Palaeoproterozoic age occur within the outcrop of the Loch Maree Group and also within the Archaean gneiss complex, cutting the 'Scourie' dykes. Since they post-date either the dykes or the c. 2.0 Ga Loch Maree Group (LMG), they are considered to be Laxfordian in age. They are broadly granitoid in composition, and fall into three categories: (a) a suite of gneissose granodiorites and tonalites, including the Ard gneiss and Mill na Claise gneiss, which intrude the LMG; (b) thin granite sheets cutting both Archaean basement and 'Scourie' dykes; and (c) granitoid pegmatites.

5.1 The Ard-type gneisses 5.1.1 Distribution and structural relationships The Ard gneiss sensu stricto (Park 1964) is a gneissose granodioritic to tonalitic body (Fig. 5.1 A) whose outcrop extends from the type locality on the An Ard peninsula [802 751], where it is 600m wide, southeastwards to Dubh Loch where it narrows to about 150m. Similar gneisses, which are grouped with the Ard gneiss for convenience, occur in the core of the Mill na Claise fold (the Mill na Claise gneiss), and form a narrow belt along the SW side of the Mill na Claise amphibolite from Shieldaig Lodge to the SW end of Dubh Loch (the Cloiche gneiss) (see map). All these gneisses enclose numerous sheet-like bodies of amphibolite similar to, though coarser than, the amphibolites associated with the LMG metasediments. In the NW, on An Ard peninsula and NE of Lochan Dubh nan Cailleach, these amphibolites form distinct sheets up to 150m wide. Further SE, at Druim Ruadh [822 725] three of these sheets unite to form a large mass of amphibolite, 300m across, which interfingers with the gneiss on its SE side and eventually disappears southeastwards. Where the Ard gneiss outcrop crosses the main road, only a single thin band of amphibolite is present. These amphibolite sheets are intruded by quartzo-feldspathic veins and by thin sheets of granitic material similar to the Ard gneiss (Fig. 5.1B), indicating that the protoliths of the Ard gneiss were intrusive into the rocks of the LMG. This veining is not found in the LMG amphibolites further NE. The contact between the Ard gneiss and the metasediments on both the NE and SW sides is highly sheared, and its original nature obscured. Thin bands of amphibolite also occur within the Mill na Claise gneisses, and two of these bodies are folded around the hinge of the Mill na Claise fold. These amphibolites were mapped as Scourie dykes by Clough (see Geological Survey of Great Britain, sheet 91) since the gneisses were then thought to be part of the 'Fundamental Complex'. However their relationship with the gneisses is ambiguous; there is no clear evidence of discordant relationships and the sheets do not show the typical sub-ophitic texture of the Scourie dykes, and so they may have the same origin as the other LMG sheets within the Ard gneiss. In addition to the discrete mapped bodies of amphibolite, the gneisses contain numerous small elongate inclusions of mafic material, typically only a few centimetres in width. The Cloiche gneiss also encloses several bands of meta-limestone (see Section 4.3). The Ard gneiss is thoroughly deformed and partly or completely recrystallized (Fig. 5.1 A); in some places it is intensely sheared, notably along its contacts with the metasediments. However the meta-igneous nature of the gneiss is obvious over much of the outcrop from its compositional homogeneity and the ubiquitous presence of deformed and recrystallized megacrysts, now generally in the form of augen. The nature of the gneisses can be conveniently studied at the type locality of An Ard, in a series of excellent coastal

exposures both along the north coast at, and just west of, An Dun [802 754], and along the south coast, west of Gairloch pier. The strongly deformed gneisses possess a penetrative NWtrending gneissosity and steeply NW-plunging lineation. The original feldspar megacrysts are recrystallized and deformed into augen which are elongated parallel to the lineation. The deformation increases in intensity towards the junction with the metasediments. There is an abrupt change from moderately SE-plunging D2 lineations in the biotite schists of the Charlestown belt to the steeply NW-plunging lineations in the adjoining Ard gneisses. The fabric generally displays an upper greenschist facies assemblage, which is clearly retrogressive; however hornblende is stable in the enclosed amphibolites, which preserve typical amphibolite-facies metamorphic assemblages. Both the amphibolites and, to a lesser extent, the gneisses, exhibit small-scale tight folds similar to the F2 (Laxfordian) folds of the metasediments. The granodioritic and quartzo-feldspathic veins found in the southwestern amphibolite sheet are affected by these folds (Fig. 5.1B), evidence that the granitic material was intruded into the amphibolites before the folding and foliation developed in the gneiss. In the amphibolites, these folds affect an earlier foliation attributable to S\ but this early foliation is not evident in the gneisses, prompting the observation that the protolith of the Ard gneiss may have been emplaced after S1 and before D2, and thus be syntectonic with respect to the Early Laxfordian deformation (see Section 6.2.6). However, it is equally possible, that the S1 foliation was more weakly developed in the Ard gneiss compared with the adjoining metasedimentary schists and may have been obscured by the more strongly developed D2 and D3 structures. 5.7.2 Petrography Three distinct petrographic types of gneiss can be distinguished. The main outcrop of the Ard gneiss, which extends from An Ard southeastwards along the coast to Loch Kerry, consists of uniform, dark grey, gneissose granodiorite with conspicuous small pink feldspar augen, or lensoid feldspar aggregates, of the order of 23mm across (Fig. 5.1 A). The northeastern part of the outcrop of the Ard gneiss, between and NE of the thick amphibolite sheets, comprises a more tonalitic grey gneiss with typically narrow, elongate, felsic lenses rather than augen. The Mill na Claise gneiss is characterized by larger feldspar augen, up to 1 cm or more across and, unlike the other gneisses, contains significant amounts of muscovite. The Cloiche gneiss contains small augen similar to the Ard gneiss. The granodioritic augen gneisses consist of 40-50% plagioclase, 20-30% quartz, 10-25% greenish-brown biotite, and 5-10% microcline, together with minor amounts of epidote, and accessory apatite, titanite, and opaque ore. In some samples, the biotite is extensively altered to green chlorite. Muscovite occurs in significant amounts (5-10%) only in the Mill na Claise gneiss. Replacive calcite occurs in some samples. The augen represent original plagioclase megacrysts completely recrystallized either to granular aggregates of plagioclase and minor quartz, or to large albite porphyroblasts seived with euhedral epidote. The augen are wrapped by biotite. Many samples show considerable granulation along the boundaries of the augen. The mean grain size of the recrystallized mosaics is typically around 0.lmm although the original igneous grain size was considerably larger, perhaps around 1mm. The augen may be up to 5mm or more in diameter. The even-grained tonalitic variety of the Ard gneiss typically exhibits a well-developed planar biotite fabric and is more pervasively recrystallized than the granodioritic variety, with a

46

CHAPTER 5

Fig. 5.1. (A) Homogeneous granodiorite gneiss with small feldspathic augen and thin quartzofeldspathic stripes, Ard gneiss. An Ard [803 753]. (B) Granodiorite (Ard gneiss) veins intruding LMG amphibolite, An Ard. Note steeply plunging lineation (e.g. to right of hammer head) [803 753].

mean grain size of around 0.2mm and flattened feldspathic augen up to 2mm across. Typical examples contain 30-35% plagioclase, 20-25% quartz, 25-30% biotite, and around 10% epidote. Calcite is commonly found in addition, and accessories are the same as for the granodioritic augen gneisses. The Mill na Claise gneiss typically contains large feldspathic augen, around 1 cm across, recrystallized to a granular mosaic with individual grains up to 2mm in diameter, in a much finer-grained matrix with a mean grain size of 0.1-0.2 mm. Muscovite is the

characteristic mica and the small amounts of biotite are typically partially or completely replaced by chlorite. Otherwise these gneisses are mineralogically similar to the granodioritic augen gneisses of the Ard gneiss. The transitional zone between the Ard gneiss and the Charlestown schist to the NE is exposed along the foot of the raised sea cliff at the side of the approach road to the Gairloch pier, at [807 751]. It is about 20m wide and consists of intensely sheared gneisses that appear similar to the metasedimentary schists except for the

LAXFORDIAN INTRUSIVE ROCKS

presence of small augen. In thin section, these rocks have a protomylonitic texture. There is some interbanding of metasedimentary and meta-igneous material within this zone but because of the high degree of deformation, the two rocks are difficult to distinguish from each other. Winchester et al. (1980) analysed a number of samples of these 'transitional' schists from several localities along both sides of the Ard gneiss outcrop and concluded that they were chemically distinct from the metasedimentary schists but similar to the meta-igneous Ard gneiss.

5.1.3 Geochemistry Eighteen major-element analyses by Holland & Lambert (1973) and 11 unpublished analyses by the author (Park 1963) are summarized in Table 5.1. A comparison of the tonalite gneisses (column A) with the granodiorite gneisses of the main Ard gneiss outcrop (column B) shows, as expected, that the latter are characterized by higher SiO2 and K2O values and lower FeO. The Mill na Claise gneiss (column C) is similar, but more silica-rich. Rare-earth element (REE) and multi-element plots (Park et al. 2001; Fig. 5.2) show significant heavy rare-earth and Y depletion characteristic of TTG (trondhjemite-tonalite-granodiorite) suites in the Archaean and adakitic magma compositions in the Phanerozoic (Defant & Drummond 1990; Martin 1993). Moreover they show other characteristics of these magma types (cf. Tarney & Jones 1994) such as very high Sr and Ba, high K/Rb ratios, prominent Nb and Ti troughs and no significant Eu anomalies. On discrimination diagrams (e.g. Rb v. Nb/Y) they have a primitive arc signature that is supported by their initial 87Sr/86Sr ratio of 0.7029 at 1.90 Ga. (Bridgwater et al. 1997). This suggests an important juvenile Proterozoic component added to the Gairloch sequence at c. 1.90 Ga. These REE and multi-element patterns differ so fundamentally from those of the metasediments of the LMG (Fig. 4.5) that the involvement of Gairloch metasediments in the petrogenesis of the Ard Gneiss must have been negligible, a fact confirmed by the more radiogenic Sr ratios of the metasediments (Bridgwater et al. 1997).

5.1.4 Geochronology A precise U-Pb zircon age of 1903 + 3-2 Ma for the Ard Gneiss is provided by Park et al. (2001) from a sample collected from fresh exposures behind the fish factory 150 m west of Gairloch pier [805 750]. The sample is a typical granodiorite augen gneiss containing quartz, plagioclase, K-feldspar, biotite, and epidote, and yielded subhedral to euhedral elongate (3: 1), clear, colourless zircons that range in size from 50 to 150 um. Cathodoluminescence images of zircons from this population shows zoning consistent with an igneous origin and the zircon age is interpreted to represent the crystallization age of the Ard Gneiss and hence may be used to estimate of the age of the Laxfordian D1/D2 event (see Section 6.1.1).

5.2 Granite sheets Thin, pink-weathering, microgranite sheets are found in a few places in the hinge zone of the Tollie antiform and at Creag Mhor Thollaidh. The sheets cut both the Scourie dykes and their host gneisses, and are affected by Laxfordian deformation; they are thus of Palaeoproterozoic age. Good examples may be seen in the small cliff at [844 779], overlooking the south shore of Loch Tollie. Here the sheets are between 0.3 and 0.5m in thickness and occur within, and sub-concordant with, a thick, gently-inclined, Scourie dyke. They contain a well-developed planar and linear fabric similar to that in the enclosing dyke. The sheet consists mainly of plagioclase, quartz and biotite and has a mean grain size of about 0.3-0.4 mm.

47

Table 5.1. Chemical analyses (major elements) of Ard-type gneiss

SiO2 Ti2O3, A12O3 Fe2O3 FeO MgO CaO Na 2 O K2O n

A

B

C

D

62.43 0.52 16.16 1.71 2.35 2.68 4.22 5.17 1.83

65.78 0.41 15.95 1.78 1.39 2.25 3.35 5.32 2.51

71.49 0.23 14.92 0.78 0.71 0.75 2.07 5.96 2.32

63.8 0.53 16.6 4.6t

4

4

3

18

3.0 4.2 5.2 2.0

A-C from Park (1963), D from Holland & Lambert (1973). A, Mean of four samples of tonalite gneiss from NE zone of Ard gneiss outcrop. B, Mean of four samples of granodiorite gneiss from main Ard gneiss outcrop. C, Mean of three samples of granodiorite gneiss from the Mill na Claise gneiss. D, Mean of 18 samples of Ard-type gneiss. t Total iron

Another example, from Creag Mhor Thollaidh, contains hornblende, epidote, and chloritized biotite in addition to quartz and plagioclase, and is more trondhjemitic in composition. Titanite, apatite, and opaques occur as accessories.

5.3 Granitoid pegmatites

5.3.1 Distribution and petrography Thin pegmatite and aplite veins cut the Ard gneiss, where they are folded and foliated with the host gneisses. These bodies are typically granitic, containing microcline, plagioclase, quartz, and muscovite, although more trondhjemitic varieties containing hornblende and biotite are found within the accompanying amphibolite sheets. They fall in a similar age bracket to the Ard gneiss itself, as they are affected by the Laxfordian D2 deformation. The pegmatites probably mark the waning phase of igneous activity represented by the Ard gneiss. Granitic pegmatites are widespread in the Archaean gneisses NE of the Gairloch schist belt, notably in the crestal region of the Tollie antiform. These pegmatite bodies are either sheet-like, lensoid, or irregular in form and clearly cross-cut both the Scourie dykes and the Laxfordian D2 fabric within the Scourie dykes. In the crestal region and NE limb of the antiform, they are either completely undeformed, or are locally affected by a cataclastic deformation which produces a crude planar fabric. In the SW limb of the antiform, the pegmatites are strongly affected by this fabric (Laxfordian S3), which is sub-parallel to the NW-trending, steeply dipping foliation of the host gneisses. The Tollie pegmatites are typically pink-weathering and coarsegrained, and vary from less than 1m to about 10m in thickness. Some are of mappable size; for example in the area east and north of Meall an Spardain, linear bodies with various orientations occur at [856 769], [861 767], and [856 762]. To the south of these localities, near the summit of Meall an Doirean, several elongate lensoid masses occur (see map). However, most examples are too small to be represented on the map. The pegmatites vary widely in mineralogy but typically contain both potash feldspar and plagioclase (K-feldspar being dominant) together with quartz, and small amounts of muscovite and epidote. The highly deformed pegmatites in the SW limb of the Tollie antiform contain bands of highly deformed quartz mosaic enclosing lenses and augen consisting of large feldspar grains. The coincidence of pegmatite occurrences with the crestal region of the Tollie antiform (and also in the Carnmore antiform further north) was noted by Clough (in Peach et al. 1907) who implied that

48

CHAPTER 5 Table 5.2. Chemical analyses of post-D 2 Laxfordian granitoid pegmatites from Holland & Lambert (1995). A

SiO2 A12O3 Fe2O3t MgO CaO

Na 2 O K2O

TiO 2 MnO La Ba Nb Zr Y Sr Rb Zn Cu Ni Li

n

70.1 16.3 0.5 0.2 0.5 2.5 0.01 0.06 0.011 36 620 12 46 18 174 676 7 10 3 3 16

B

73.5 15.0 1.0 0.3 1.1 3.7 5.3 0.08 0.012 35 1210 41 69 4 353 131 14 19 4 7 33

A. Mean of 16 analyses of granite pegmatites from the Tollie area. Gairloch. B, Mean of 33 analyses of granitoid pegmatites from the Laxford area (Northern Region). 'Total iron

Fig. 5.2. (A) Chondrite-normalized REE plot of Ard gneiss samples. (B) Mantle-normalized multi-element diagram showing average Ard gneiss compared with average Gruinard tonalite and trondhjemite. A,B, from Park et al (2001).

their occurrence there was because deeper crustal levels were exposed in the antiformal hinge zones. Granitoid pegmatites also occur within the LMG, in association with the meta-limestones of the Shieldaig marble belt. For example at the marble quarry near Shieldaig Lodge, at [812 723], a pegmatite sheet, several metres thick can be followed for 250 m along the SW side of the carbonate band, and has been mapped again alongstrike to the SE at Airigh na Cloiche. This pegmatite contains plagioclase, quartz, and muscovite, and is partly deformed, with thin granulated zones around larger feldspar grains and sutured quartz mosaics. Although these bodies are certainly Laxfordian, they may be either pre-D2 or post-D2, pre-D3.

5.3.2 Geochemistry Holland & Lambert (1995) carried out major and trace-element analyses on 16 samples of pegmatite from the crest and SW limb of the Tollie antiform. The composition is distinct from that of all

other mainland Lewisian pegmatites in having high K and Rb, unusually low K/Rb ratios, relatively low Ba and Sr. and low levels of Li. The mean composition of these pegmatites is compared with the average of 33 pegmatites from the Laxford area in Table 5.2. Holland & Lambert (1995) suggest that the pegmatites could be the products of shear-enhanced partial melting of the granodioritic Tollie gneisses, or alternatively that they may be the product of fractional crystallization of a more highly differentiated material than is normally seen in the Lewdsian gneiss complex and which is no longer represented at the surface. The former explanation is considered more likely, i.e. that these bodies were emplaced at. or shortly after, the climax of the main Laxfordian metamorphism, after the Laxfordian D 2 deformation had ceased, but prior to the Laxfordian D 3 deformation.

5.3.3 Geochronology Holland & Lambert (1995) also carried out Rb-Sr isotopic analyses on a set of 13 samples of pegmatite, but the data are difficult to interpret. A selected 9-point isochron was taken to imply a crystallization age of 1663 2 Ma with an initial 87Sr 86Sr ratio of 0.705, coincident with that of the Tollie gneisses. Some of the samples were taken from the more deformed bodies on the SW limb of the antiform but there is no correlation between apparent age and amount of deformation. Holland & Lambert considered that the scatter of points may be the product of an open isotopic system involving variable loss of 87Sr into the host gneisses. If this were the case, and if there were no comparable loss of Rb, the upper boundary of the data spread (i.e. the 9-point 1663 Ma isochron) would be expected to give the closest approximation to the age. However, even if this 'isochron" age can be relied upon, it probably indicates the date of the D 3 deformation, rather than the original crystallization age of the pegmatite. Subsequently, a precise zircon date of 1694 5 Ma was obtained from a pegmatite collected from the top of the hill immediately

LAXFORDIAN INTRUSIVE ROCKS

south of Loch Tollie, at [846 778] (Park et al 2001). The pegmatite sample is microcline rich with plagioclase and muscovite and yielded subhedral to euhedral, elongate zircons (4:1) that range in size from 25 to 125 um in length and vary in colour from cloudy and white to clear with a red coating. Three fractions define a line with intercepts of 1694 5 Ma and 436 11 Ma, and in view of the

49

igneous morphology of the zircons analysed, the upper intercept is interpreted to represent the crystallization age of this pegmatite dyke. The 1694 Ma date provides a lower limit to the age of the Laxfordian D 1 / D 2 event and an upper limit to the Laxfordian D 3 event (see Section 6.1.2).

Chapter 6 Laxfordian Structure and Metamorphism 6.1 Introduction The key to the Lewisian structural chronology in Gairloch, as elsewhere in the Lewisian complex, is the Scourie dyke swarm, which, following Peach et al (1907) and Sutton & Watson (1951), has been used to divide the complex into pre-dyke and post-dyke elements; the post-dyke structures and events are referred to the Laxfordian. However, as explained in Chapter 3, none of the dykes attributed to the Scourie dyke swarm in the Gairloch area, or in the Southern Region generally, have been dated (except for K-Ar dates interpreted as metamorphic cooling ages). Therefore they may correlate with either the 2.4 Ga or 2.0 Ga dykes of the Central Region (see Section 1.2.6), or could represent a third swarm of some other age. It is also possible that dykes of two or more different ages are represented. While the later Laxfordian structural and metamorphic elements can be correlated throughout the Lewisian complex, some of the earlier post-dyke structures assigned here to the Laxfordian may not correspond to similar structures elsewhere in the complex if the dykes themselves are not of the same age (see discussion on chronology in Section 1.3). Structures are also assigned to the Laxfordian if they affect the c. 2.0 Ga rocks of the Loch Maree Group (LMG). Although the Archaean gneisses share in the Laxfordian deformation, the following structural analysis depends critically on the evidence from the dykes and the LMG.

K-Ar hornblende ages considered to relate to the retrogressive events fall into two groups, one clustering around 1.7 Ga and the second around 1.5 Ga, ranging down to c. 1.4 Ga (Moorbath & Park 1971). The 1.7 Ga ages can probably be regarded as confirming the age of the regional D3 retrogression and the 1.5 Ga cluster as giving a rough estimate of the age of the more localized D4 deformations. Within the central part of the Gairloch district (including the outcrop of the LMG), it is difficult to determine the nature and extent of the earlier structures (D 1 /D 2 ) because of the very intense late Laxfordian deformation (D3), which has resulted in the nearparallelism of all rock units and structures. Within this central highly deformed belt, most outcrops show an intense and pervasive NW-striking, steeply dipping, foliation which is axial planar to minor, tight to isoclinal, folds. These structures are developed in association with amphibolite-facies metamorphism as indicated by the ubiquitous presence of aluminous hornblende and calcic plagioclase in the basic rocks. They are attributed mainly to the first and second Laxfordian deformations (D1 and D2), which cannot be separated easily in this area. The third Laxfordian deformation (D3) is associated with the development of the Gairloch shear zone and with several major, upright, NW-trending folds. Minor folds of this generation are associated with retrogression to greenschist facies. Later deformation (Laxfordian D4) has produced localized minor folds with steep plunges, and narrow brittle-ductile shear zones containing cataclastic breccias and pseudotachylite.

6.1.1 Structural overview Laxfordian deformation in the Southern Region of the mainland Lewisian is distributed through a wide belt over 25 km across (see Fig. 1.3), in which the Laxfordian deformation is developed in a very heterogeneous manner. Numerous variably-inclined NWtrending zones of localized high strain are separated by areas of lower strain. The high-strain zones have in many cases been folded and re-orientated by the younger Laxfordian deformation. Localization or concentration of Laxfordian structures has been promoted by the heterogeneity of the pre-existing structure of the complex; in particular, the presence of the dykes and the rocks of the Loch Maree Group have, to a varying extent, focused the Laxfordian strain. The nature of the basement gneiss complex has also had an effect on the development of Laxfordian structures, which are concentrated in areas previously affected by the more favourably-orientated Inverian structures. 6.1.2 Deformation phases and chronology Four phases of Laxfordian deformation (Laxfordian D1-D4) have traditionally been recognized (e.g. see Park et al. 1987 and Table 1.1). D1 and D2 are associated with an amphibolite-facies metamorphic event. D3 and D4 are associated with retrogressive metamorphic events, D3, at low amphibolite to greenschist facies, affecting the wide region of the Gairloch shear zone, and D4, more localized along narrow belts, at sub-greenschist facies. The Laxfordian D 1 /D 2 high-grade event has a minimum age of c. 1.7 Ga from dating of both metamorphic and late-tectonic igneous events (see Section 1.2.8), and may correlate with the 1.87 Ga Sm/ Nd metamorphic event recorded in South Harris (see Cliff et al. 1983, 1998). In Gairloch, the zircon age of 1.90 Ga for the emplacement of the Ard gneiss (Park et al. 2001) can be regarded as the likely maximum age of the D1/D2 event. An estimate for the age of the retrogressive D3 assemblages is provided by the zircon age of 1694 Ma obtained on a pegmatite which cuts the D2 structure in the Tollie antiform but is deformed by the F3 deformation (Park et al. 2001; see Section 5.3.3).

6.1.3 Metamorphic conditions The metamorphic conditions accompanying the D2 deformation have been investigated by Droop et al. (1998) in the LMG rocks of both Gairloch and Loch Maree. (Droop et al. do not recognize the D1 event of this work, so that the D2 event corresponds to their D1). Droop et al. show that the main phase of Laxfordian metamorphism (i.e. syn-D2) reached amphibolite facies, as noted by the earlier workers. They concluded that the peak D 2 metamorphic assemblages had grown statically, but that the later D2 movements within the mylonite belts (their DM phase) took place under decreasing temperature. They also point to a variation in peak metamorphic conditions between almandine-amphibolite facies at Loch Maree and epidote-amphibolite facies at Gairloch. However, as noted in Section 4.9, there appears to be a variation in the peak metamorphic assemblages even within the Gairloch area, for example between the northeastern and southwestern LMG amphibolites. Droop et al. (1998) suggest a peak metamorphic temperature at Gairloch of 530 20°C, based on calcite-dolomite thermometry in marbles and daphnite-dehydration equilibria in banded iron formation, and a pressure of c. 6.5 1.5 kbar, based on garnetplagioclase equilibria. They conclude that the D2 event was associated with tectonic thickening of the crust, in accord with the accretion/collision model proposed here (see Section 7.4.1).

6.2 Early Laxfordian (D1/D2) The early Laxfordian structures (Di/D 2 ) occur in several distinct environments, which are described separately below. 6.2.1 Narrow marginal shear zones in Scourie dykes Marginal shear zones occur in steep NW- to WNW-trending dykes (Park et al. 1987) in the form of narrow foliated zones defined by amphibolite-facies fabrics, in places accompanied by steep NW-

52

CHAPTER 6

plunging elongation lineations. Such zones are preserved both in the SW part of the Gairloch area, near Loch Braigh Horrisdale (e.g. Fig. 6.1), and in the NE, on Creag Mhor Thollaidh. Many good examples are seen in the dykes immediately north and east of Loch Braigh Horrisdale. The sense of shear determined from the obliquity and curvature of the fabrics in a large number of dykes in this area and in the Creag Mhor Thollaidh block is mainly oblique sinistral and SW-side up. Similar structures occur in the dykes outside the map area, to the south, towards Torridon, and NE of Loch Maree, between Creag Mheall Beag and Gruinard Bay. In the latter area, sinistral shears are accompanied by a complementary set of dextral, south-up shears in dykes with a NNE orientation (Park el al. 1987).

Fig. 6.2. Photomicrograph of intensely deformed (schistose) Cloiche gneiss showing recrystallized feldspar augen with tail indicating dextral shear sense and aligned biotites defining S 2 fabric: width of field. 32mm ( x 2 ) : [813 724].

Fig. 6.1. Sketch of marginal shear zone in amphibolite Scourie dyke showing asymptotic bending of marginal foliation with a dextral shear sense. The central part of the dyke is unfoliated, Sidhean Mor [812 716].

6.2.2 SW of the Loch Maree Group outcrop As the highly deformed LMG belt is approached from the south, the marginal foliations in the dykes are progressively replaced by a more intensely developed fabric, with a moderately SE-plunging elongation lineation, indicative of considerably higher strains (Park et al. 1987). In a belt about 700m wide, south of Shieldaig, the dykes are rotated from a steeply SW-dipping attitude (generally 7080°SW) to an average dip of 70-80°NE. The fabric is defined by amphibolite-facies mineralogy; the dykes have recrystallized to a hornblende-calcic plagioclase assemblage. The dykes are pervasively deformed and the host gneisses also exhibit this fabric, which is finer-grained, with more closely spaced foliation planes, than the Inverian banded gneissose fabric that it replaces. A sinistral, NE-up sense of movement was determined from the dyke fabrics in this zone. However, in the southwestern part of the Gairloch supracrustal belt, within the Cloiche marble belt, a dextral, south-up, shear sense was determined from the asymmetry of feldspar augen in highly deformed (schistose) Cloiche gneiss (Fig. 6.2) and deformed garnets in garnet-schist bands. The garnets are strongly elongated parallel to the SE-plunging L 2 lineation, and are broken and partly replaced by biotite.

6.2.3 Supracrustal rocks of the Gairloch schist belt The dominant fabric in the metasediments and metabasites of the LMG is a pervasive, intensely developed foliation, typically accompanied by a strong linear fabric (Fig. 6.3) marked by elongation of grain aggregates (typically quartz), mineral orientation (e.g. hornblende), and mica intersection (e.g. see Fig. 4.2A).

Rare intrafolial folds (F1) occur, mainly in the more siliceous lithologies such as banded iron formation (Fig. 6.4). F2 microfolds that fold an earlier fabric (S 1 ) and are coaxial with the strong lineation are common (Figs 6.4C 6.5). Recrystallization in the hinge zones of these microfolds has resulted in amphibolite-facies assemblages, and this fabric is therefore regarded as a composite D1 -D2 fabric. There are no signs of retrogression, unlike the later folds." This pervasive, high-strain, LS fabric is found throughout the supracrustal belt. The foliation dips uniformly steeply, between 65°NE and vertical (average 70-75°NE) and the lineation plunges typically at moderate angles (35-55°) to the SE (average 45°SE). Exceptions to this pattern occur around the closure of the Mill na Claise fold, where the lineations are near-vertical, parallel to the plunge of the fold hinge, and in the adjoining Ard gneiss outcrop, where they plunge steeply to the NW. These differences are discussed below (Section 6.2.8). In the northeastern part of the Aundrary amphibolite, the lineations also plunge NW. possibly due to the effects of the younger D3 deformation (see below r ). Good examples of D1. D2. and D3 structures can be seen at [818 748] in the Flowerdale valley, at a low ridge composed of banded iron formation. The rock is intensely deformed, with a steep. NWtrending foliation, isoclinal, intrafolial. F 1 folds (Fig. 6.4A) and a prominent SE-plunging L 2 lineation. The L 2 lineation appears to be superimposed on the isoclinal folds. At the southeastern end of this ridge, near the edge of the trees, this foliation together with the associated lineation and isoclinal F1 folds, is bent around a small F3 fold, about 1 m across, which plunges about 30°SE. Another good locality is situated immediately below the Gairloch-Kinlochewe road at [840 720]. where sparse intrafolial F1 microfolds occur in Kerrysdale siliceous schists, which are also affected by numerous small-scale, asymmetric F2 folds that fold the S1 schistosity and plunge SE parallel to the L 2 quartz elongation lineation. In the northeastern part of the outcrop of the Flowerdale marble belt, at [825 736], a narrow shear zone, about 20m wide, contains both brittle cataclastic rocks and more ductile mylonites. The mylonitic rocks include a highly deformed fragmental rock consisting of flattened and elongate fragments of quartzite. up to several centimetres across, together with minor quantities of calcareous and chloritic schist, in a matrix of calcareous chlorite schist. The brecciation pre-dates the ductile shearing and the rock is thought to represent a pre-metamorphic decollement zone (see Section 4.6.1). Very high prolate strains are indicated by the shapes of the quartzite fragments, with typical axial ratios of X : Y: Z = 50:2: 1. The elongation lineation plunges at 50-70° to the SE.

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Fig. 6.3. L2 elongation lineation in quartz-biotite schist, Kerrysdale schist [833 727].

Fig. 6.4. D1 and D2 minor structures. A, Tight to isoclinal intrafolial Fl folds of siliceous bands in banded iron formation, Flowerdale [819 746]. B, Tight ptygmatic folds of quartz vein in semipelite schist with prominent S2 foliation, Flowerdale schist [826 740]. C, Isoclinal Fl folds refolded by F2, in banded iron formation, Flowerdale [817 750].

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Sense of movement criteria have been studied in two mylonite belts (see Lei & Park 1993), the Flowerdale shear zone referred to above, and the Torr an Easain shear zone. The latter zone occurs in siliceous schist and banded iron formation within the Kerrysdale amphibolite, about 300m SW of the Flowerdale shear zone, and can be traced for about 1 km. The mylonitic banding in both these shear zones appears to be equivalent to the D2 fabric in the surrounding schists and displays the same amphibolite-facies assemblage, being marked by the alignment of minute grunerite needles in the banded iron formation. The sense of movement parallel to the strong linear (L2) fabric in these mylonite belts is sinistral, NE-up, as indicated by asymmetric shear folds, shear bands and extensional cleavage (ecc) structure (Lei & Park 1993) (see Fig. 6.15A).

6.2.4 The NE limb of the Tollie antiform

Fig. 6.5. Photomicrograph of F2 fold hinge in quartz-hornblende schist, Flowerdale marble belt, Flowerdale, showing recrystallized hornblende crystals; width of field, 64mm (x2).

On the NE limb of the antiform. the dykes, together with the Inverian foliation in the host gneisses, have been rotated into a gently to moderately dipping attitude (Fig. 6.6). The dykes contain a concordant or sub-concordant Laxfordian LS fabric, defined by an amphibolite-facies mineral assemblage. A strong intersection elongation lineation plunges SE. A number of strain measurements were made using estimates of the shapes of ellipsoidal felsic grain aggregates in the dykes (cf. Coward 1976). All the measured strains

Fig. 6.6. Schematic diagram representing the structure of the Tollie antiform, showing the following features (see text for further explanation); from Park (1970). A, Overall profile as seen from the north, e.g. from the Gairloch-Poolewe road. B, Detail from gently-dipping NE limb showing upright Inverian folds refolded by Laxfordian F2 folds and cross-cut by gently-dipping S2 foliation. C, Detail from NE limb showing gently-dipping dyke with marginal Si foliation and affected by sub-horizontal S2 foliation. D, Generalized profile showing stepped nature of dykes on NE limb and upright F3 folds on SW limb. E. Series of profiles from NW to SE showing progressive southeastwards tightening of main fold hinge and of F3 satellite folds.

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Fig. 6.7. Simplified map and cross-section of the Tollie antiform showing the change in dyke pattern and orientation from the Creag Mhor Thollaidh block in the east to the NE limb of the Tollie antiform. Note the stepped pattern of the dykes in cross-section, with alternate sections discordant to, and concordant with, gently-dipping S2; A-A', axial trace of Tollie antiform; from Park (1970).

were prolate, with X: Z ratios varying from 4:1 to 13:1. The dykes are divided into sectors that are alternately discordant to, and sub-parallel with, the sub-horizontal fabric (Fig. 6.6D, 6.7). The concordant sectors are more highly deformed and exhibit a well-developed foliation, whereas the steeper discordant sectors exhibit a mainly linear fabric. The sense of shear implied by these more highly strained sectors has a top to the west component (Fig. 6.7), implying in turn that the overall shear sense is towards WNW. Generally only a single fabric is seen in the dykes, but occasional examples have been found where an earlier, margin-parallel, foliation (S1) is folded by folds whose axes are parallel to the strong lineation. Hence the strong fabric is assigned to D2. This structure corresponds to that clearly displayed in the dykes of the Torridon area (Niamatullah & Park 1990), where the high-strain event attributed to D2 has resulted in the rotation of the dykes into their present gently inclined attitude.

6.2.5 The western margin of the Creag Mhor Thollaidh block Steep NW-trending dykes and Inverian-deformed banded gneisses along the western edge of the Creag Mhor Thollaidh block are affected by steep SE-plunging folds and rotated into a NE-trending attitude (Fig. 6.7). As in the case of the Tollie antiform to the west, the dykes were recrystallized under amphibolite-facies conditions, and the fabric at the fold hinges is strongly linear. This deformation is ascribed to D2. The precise geometric relationship to the Tollie antiform structures has been disrupted by displacement on the intervening Leth-chreige crush belt. At the northern end of the block, several dykes, for example at [862 779], are concordant with the NE-SW-trending Scourian gneissosity. These dykes do not appear to have been strongly deformed, but commonly possess a well-developed linear L2 fabric, and in places a margin-parallel S1 foliation is folded by minor F2 folds with steep, NW-trending, axial planes and cut by an axial

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Fig. 6.8. F2 folds of granodiorite vein in amphibolite, Ard gneiss, Druim Ruadh [832 725]: penknife is 8 cm long.

plane S 2 foliation. Confusingly, these D 2 structures are sub-parallel to the earlier Inverian structures.

6.2.6 The Ard gneiss The Ard gneiss, in the southwestern part of the LMG belt, possesses a penetrative NW-trending gneissosity and steeply NWplunging lineation attributed to D2 (see Fig. 5.1 A). The original feldspar megacrysts are recrystallized and deformed into augen that are elongated parallel to the lineation. Several of the well-foliated amphibolite sheets enclosed within the Ard gneisses contain granodioritic and quartzo-feldspathic veins affected by tight F2 folds (Fig. 5.1B, Fig. 6.8). The deformation increases in intensity towards the junction with the metasediments. There is an abrupt change from moderately SE-plunging D 2 lineations in the biotite schists of the Charlestown belt to the steeply NW-plunging lineations in the adjoining Ard gneisses. The gneisses generally display retrogressive upper-greenschist facies fabrics but hornblende is stable in the enclosed amphibolites. Although the S1 foliation is folded by the small-scale F2 folds in the amphibolites (Fig. 6.8), there is no sign of pre-F2 structures in the gneisses. The igneous protoliths of the gneisses may have been intruded after the supracrustal complex had already been deformed in D 1 ; however, the D 1 structures may not have formed in the igneous body due to its more competent nature.

6.2.7 Relationship between D1 and D2 There is no evidence of a significant temporal separation between the first fabrics (S1) in the dykes and the high-strain (D 2 ) fabrics in the rotated dykes of the Tollie antiform. In the light of this, Park et al. (1987) suggested that D1 and D2 may represent a single progressive deformation where early-formed steep D1 structures were subsequently rotated into more gently-dipping orientations due to progressive strain in a major low-angle shear zone (D 2 ) (see Section 6.3). The movement direction during the D2 shearing in Gairloch appears to have been WNW-ESE with a mainly top to WNW shear sense. The SW-up shear sense attributed to the dykes in the SW of the area (see Section 6.2.1) was thought to be the result of synthetic shear in the dykes as they were rotated in the footwall of the main D2 shear zone (see Fig. 6.17A)

However a complicating factor is that the complementary shear set in the dykes south of the Gruinard River appear to have formed during NE-SW compression (Park et al. 1987). Therefore, some of the dyke shears ascribed to D 1 in the Gairloch area may belong to this complementary set as well, and the 'D 1 dyke shears may not all have the same origin or be the same age. Another problem is the correlation of D 1 in the LMG with D 1 in the dykes, in view of the probability that the first deformation in the LMG preceded the collision event that brought the LMG and its Archaean basement together. This question is discussed further in Section 7.4.1.

6.2.8 Change in plunge of L2 in the Ard gneiss outcrop The abrupt change in L 2 plunge from steeply NW in the Ard gneisses to moderately SE in the adjoining metasedimentary schists (see map) is puzzling. There is no major fold to explain the change nor is there any evidence of one set being deformed by the other. Along the NE side of the Ard gneiss outcrop, there is a zone where highly sheared gneisses adjoin metasedimentary schists and are to some extent interbanded with them. Here, the plunge direction alternates between NW and SE. The narrow strip of metasedimentary schist along the SW side of the Ard gneiss outcrop also shows SE plunges, as do the Cloiche gneisses further southwest, where the L 2 plunge is parallel to the SE-plunging L 2 in the basement gneisses on their SW side. In both schists and gneisses. L2 is an elongation lineation as well as a fold axial direction and is interpreted as the movement direction of the major D 2 shear zones (see below). The LMG belt apparently contains several structural units, now juxtaposed, with different movement directions. This interpretation is further explored in Section 7.4.1.

6.3 Large-scale D2 structure The large-scale D 2 structure of the Gairloch area is interpreted by Park et al. (1987) as a major, originally low-angle, shear zone (see Fig. 6.17A), including the LMG outcrop, the Tollie gneisses to the NE, and part of the Shieldaig gneisses to the SW. The association of dextral SW-up and sinistral NE-up shear sense within this shear zone is apparently contradictory, and is also found in the Torridon (or Diabaig) shear zone (Wheeler et al. 1987: Niamatullah & Park

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Fig. 6.9. (A & B) upright F3 folds at crest of Tollie antiform, [856 761], showing plunge to SE; (A) viewed from NW; (B) viewed from SE. 1990). However the prevalence of a sinistral NE-up sense of movement in the early mylonites and the Tollie dykes suggests that the main D2 movement was top to WNW on planes inclined gently NE. The present steep attitude of the foliation over much of the Gairloch area is attributed to D3 refolding (see below). The dextral SW-up shear sense, on presently NE-dipping zones (e.g. in the sheared gneisses of the Cloiche belt), is therefore extensional in their present orientation. This movement may have been due to extensional collapse of overthickened crust in the later stages of the D2 event (see Section 7.4.1).

in the Gairloch district and their axes vary in orientation from subhorizontal to steeply plunging. They fall into two groups: (1) those that are associated with limited retrogression to greenschist facies and where tight to isoclinal folds display a new axial planar fabric (assigned to D3), and in places are seen to refold L2 lineations; and (2) a widely distributed set of usually open, chevron-style, folds, with steeply plunging axes, also associated with retrogressive breakdown but without any associated new fabric (assigned to D4). Many individual microfolds cannot be assigned to either phase.

6.4.1 D3 minor structures 6.4 Late Laxfordian Traditionally, the later part of the Laxfordian structural chronology in the Southern Region has been divided into two phases, D3 and D4 (e.g. Park el al. 1987). The main Gairloch shear zone and the associated Tollie antiform (Odling 1984) have been attributed to D3, together with the Torridon antiform to the south and the Letterewe synform and Carnmore antiform to the north (see Fig. 1.3). Small-scale folds that deform the S2 fabric are relatively common

Small-scale F3 folds are widespread but tend to be concentrated in two distinct structural settings: (1) the hinge zone and SW limb of the Tollie antiform; and (2) the hinge zone and NE limb of the Mill na Claise fold. The folds along the hinge zone of the Tollie antiform vary considerably in tightness from open to isoclinal (Figs 6.9, 6.10) and in style from parallel to near-similar. They are typically symmetrical in the hinge zone, becoming asymmetrical on the SW limb of the fold.

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of the LMG outcrop, is considered to form the SW limb of the Gairloch synform, which is the complementary synform on the SW side of the Tollie antiform (see Fig. 4.1); however, the hinge zone of this synform has been completely removed by displacements caused either by the D3 Gairloch shear zone or by the later D4 Creag Bhan brittle-ductile shear zone (see Section 6.5). In the zone of highest D3 strain, in the SW limb of the Tollie antiform and in the northeastern part of the Aundrary amphibolite, the elongation lineations (generally rotated L2) plunge consistently NW at moderate angles, and this direction is taken to be the shear direction on the Gairloch shear zone (Odling 1984). The Gairloch shear zone is considered to have rotated the originally gentlydipping D2 structure on the SW limb of the Tollie antiform into a near-vertical attitude in a dextral, SW-down shear sense. The Tollie antiform hinge varies in plunge from shallow NW to steep SE, giving it a sheath-fold type of geometry that may have been caused either by lateral variations in D3 displacement or by superimposition on an uneven D2 surface.

Fig. 6.10. F3 minor structures. A. Asymmetric open F3 fold with thinned steep limb on SW limb of Tollie antiform, at [858 755]. B. Open to tight F3 folds of competent bands with local development of S3 foliation in incompetent bands on SW limb of Tollie antiform, at [843 774]. C. Upright tight to isoclinal F3 folds in hinge zone of Tollie antiform, at [859 758]. D. Upright, tight F3 folds with axial plane S3 foliation, sub-parallel to steep S2 foliation, near Loch nam Buainichean, [865 732].

The open folds have no S3 fabric, but an axial planar fabric consisting of elongated quartz aggregates and aligned biotite and muscovite is associated with the tighter folds in certain places, dependent on the lithology. The D3 fabrics in the most highly strained gneisses of the SW limb of the Tollie antiform involve retrogression to greenschist facies, for example SE of Loch nam Buainichean, at [855 725]. Feldspars are replaced by an assemblage of albite, microcline, and epidote, and myrmekitic texture is widespread, but hornblende usually survives in the mafic rocks. Aligned biotite defines an axial planar fabric to small scale F3 folds in some places, such as the Tollie antiform, whereas elsewhere, for example in the Flowerdale marble belt and the Creag Bhan belt, biotite is retrogressed to chlorite. 6.4.2 D3 major structures The Tollie antiform and Gairlock shear zone. The Tollie antiform (Figs 6.6, 6.7) is the most prominent structure in the northeastern part of the Gairloch area (Peach et al. 1907; Park 1970b). This fold, which has a moderately dipping NE limb and steep to vertical SW limb, was assigned to D3 by Park et al. (1987). The plunge varies from near horizontal in the NW to over 60° in the SE, and the axial trace varies in trend from NNW to NW. L2 elongation lineations on the NE limb of the antiform plunge gently to the SE, but on the SW limb, they plunge to the NW, gradually steepening to around 45° at the margin of the LMG supracrustal belt (Fig. 6.11). The NE limb of the antiform is cut by the Leth Chreige crush belt, which separates the gneisses of the Tollie antiform from the Creag Mhor Thollaidh block to the NE. The Tollie antiform hinge line forms the NE margin of the Gairloch shear zone (Odling 1984; Park et al. 1987), which is about 6km wide, and occupies the greater part of the belt of intense Laxfordian deformation in the central part of the Gairloch area (see Fig. 1.3). Within this shear zone, deformation is generally intense, and the foliation is steeply dipping and trends NW. However, as the whole of the outcrop of this shear zone contains rocks already deformed by early Laxfordian deformation, it is difficult to distinguish clearly the later D3 structures from the earlier D1/D2 structures. A large part of this steep belt, including the whole

The Mill na Claise fold. The prominent Mill na Claise fold in the southwestern part of the shear zone is also attributed to D3. This tight to isoclinal fold closes towards the NW and has a sub-vertical plunge. The lalltaig block to the NW is enclosed by narrow brittleductile shear zones that converge and continue along the SW side of the Mill na Claise fold. Around the hinge of the Mill na Claise fold, the older S2 foliation and L2 lineation are preserved in the amphibolite and metasedimentary units; the S2 foliation is affected by small-scale F3 folding but no new fabric is developed. However in the Mill na Claise gneiss in the core of the fold, the only obvious structure is the strong S3 axial planar fabric which is associated with retrogression of the granodioritic assemblage and the growth of biotite. muscovite and epidote. The L2 lineation around the fold hinge is steeply plunging, parallel to the main F3 fold hinge. 6.4.3 D4 structures The widely distributed steeply plunging folds which deform the S2 fabric but lack any associated new fabric (Park 1964, Bhattacharjee 1968) are assigned to D4 by Park et al. (1987). F4 folds (e.g. Fig. 6.12) vary widely in style and geometry, from open to isoclinal, and from parallel to similar, dependent on lithology. They are typically associated with narrow zones of cataclasis containing crush breccias and pseudotachylite, and with the development of retrogressive mineral assemblages, containing for example pyrophyllite and stilpnomelane. The folds are typically small-scale (centimetres to metres in amplitude) but several major folds also occur, notably at Loch Bad a' Creamh, 1.5 km NW of Loch Tollie [81 79]. outside the boundary of the mapped area; this fold has a wavelength of about 1 km. Small-scale F4 folds in many places exhibit sinistral asymmetry and are particularly common within the cataclasites of the brittleductile shear zones (see below), where they have locally deformed retrogressive chlorite-bearing D3 fabrics (Fig. 6.13). They are attributed to a phase of sinistral strike-slip movements along the steep NW-SE zones of weakness created by the D3 deformation. 6.5 Brittle-ductile shear zones Five main brittle-ductile shear zones occur within the Gairloch area and have been described by Lei & Park (1993). These relatively narrow zones (typically 10-100m wide) were described as k crush belts' by Peach et al. (1907) and are so referred to here to distinguish them from the kilometre-scale ductile Gairloch shear zone. From SW to NE, these belts are: the Mill na Claise, Torr an Easain, Flowerdale, Creag Bhan, and Leth-chreige belts (Fig. 6.14). The Mill na Claise belt divides northwestwards into two branches which define the SW and NE sides of the lalltaig block and are referred to respectively as the SW lalltaig and NE lalltaig belts. All

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Fig. 6.11. Variations in orientation of principal linear and planar elements across the Tollie area; in stereograms, shaded areas are poles to foliation; dotted areas are lineations; for details, see Park (1970). In subarea 1, there is a girdle distribution of S2, and L2 plunges gently SE. In subarea 2, the main fold has tightened and the plunge has become steeper. In subarea 3, on the SW limb of the antiform, S2 is near-vertical and L2 has a mean NW plunge. Sub-area 4A is within the Leth-chreige crush zone, where S2 has a moderate ESE dip and L2 plunges SE. Sub-areas 4B and 4C reflect Scourian and Inverian structure little influenced by Laxfordian effects.

these belts exhibit both ductile and brittle types of deformation, and the brittle structures are normally seen to cut the ductile ones. The brittle material typically occupies the central parts of the broader, more ductile zones. The distinction between ductile and brittle structures here follows the usage established by Sibson (1977) and refined by Rutter (1986). Mylonitic rocks result from temperature-sensitive crystal-plastic deformation processes at relatively low strain rates, and are characterized by ductile distributed 'flow'. They are distinguished from cataclastic rocks, which are the product of pressure-sensitive elastico-frictional deformation processes at relatively high strain rates, and are characterized by discrete brittle fracturing. The ductile mylonitic rocks of the narrow shear zones represent one end of a spectrum of ductile strain that embraces most of the metamorphic rocks of the area, which exhibit a pervasive

amphibolite-facies LS fabric. The presence of pervasive regular fabric is the most useful field criterion for separating the two modes of deformation. The Gairloch shear zones trend between NNW and NW, and the foliation strikes parallel to the trend of the zones, generally with a steep dip to the NE. The mylonites and mylonitic schists and gneisses exhibit numerous criteria that can be used to indicate the sense of movement, including asymmetrical shear folds, a- and 5type rotated porphyroclasts, asymmetric augen, extensional crenulation cleavage (ecc structure) and shear-band (S-C) structure (Fig. 6.15). The mylonite zones contain one or more sub-zones consisting of various types of cataclastic rocks, including cataclastic schists and breccias, ultra-cataclasites, and pseudotachylite veins (Fig. 6.16). These cataclastic rocks exhibit asymmetric microfolds, a-type structure, rotated clasts and en-echelon vein systems. They

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Fig. 6.12. D4 minor structures. A, Sketch of variable, disharmonic F4 folds isolated and cut by brittle shears, Kerrysdale amphibolite. [811 736]. B. Diagram showing style of asymmetric chevron-style F4 folds in schist (approximately natural size). C, Sketch of steeply-plunging F4 fold cut by brittle shear, affecting S2 foliation in Aundrary amphibolite, Sidhean Mor, [832 745].

belts contain stilpnomelane and pyrophyllite, and all higher-grade minerals are partially retrogressed.

6.5.1 Early (D2) mylonitic rocks Mylonitic rocks in the Torr an Easain and Flowerdale zones have yielded limited sinistral shear-sense criteria associated with lineations plunging moderately to the southeast; i.e. the movement is sinistral, NE-up, or top to the west. At one locality, an earlier NWplunging elongation lineation is refolded by SE-plunging folds. The SE-plunging lineations are parallel to the widespread L 2 fabric within the LMG metasediments of the Gairloch schist belt, and these narrow mylonite belts represent the highest-strain zones within the broad ductile D2 shear zone that extends from the SW margin of Creag Mhor Thollaidh in the NE to the southwestern margin of pervasive Laxfordian deformation, south of Shieldaig. and includes the whole of the Gairloch supracrustal belt.

6.5.2 D3 mylonites associated with the Gairloch shear zone In the narrow shear zones bordering the lalltaig block, and also in the Creag Bhan belt, the elongation lineation has a moderate plunge to the NW, corresponding to that of the major D 3 Gairloch shear zone, and implying a dextral. NE-up. movement (Odling 1984; Park el al 1987).

6.5.3 Later movements in the zones

Fig. 6.13. Photomicrograph showing F4 microfolds affecting mylonitised chlorite schist, Flowerdale marble belt, Kerrysdale [823 742]; width of field, 16mm (x5).

generally rework or cross-cut the earlier-formed mylonites, which remain as inclusions within the sub-zone. The mineral assemblages in the ductile mylonitic rocks vary from amphibolite facies to greenschist facies, whereas the pseudotachylite-bearing breccias of the Leth-Chreige, Creag Bhan and Shieldaig

The later, cataclastic rocks of the shear zones exhibit evidence of both sinistral and dextral movement (although apparent sinistral movement senses predominate) and structures locally imply nearhorizontal movement. The cataclastic rocks were not all formed during the same movement episode. Some sub-horizontal dextral movements may have resulted from the rotation during D 3 of the pre-existing banding/foliation in the shear zones into a vertical attitude, after which further movements would have resulted in dextral strike-slip rather than oblique-slip. This change would mean that low-grade assemblages would be formed within the footwall of the shear zone as well as in the hanging wall. Cataclasites with chlorite-bearins fabrics attributed to D 3 in the

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Fig. 6.14. Outline map of the Gairloch area showing the principal ductile-brittle shear zones ('crush belts') in black. LCB, Leth-chreige belt; CBB, Creag Bhan belt; FB, Flowerdale belt; TEB, Torr an Easain belt; MCB, Mill na Claise belt; NEIB, NE lalltaig belt; SWIG, SW lalltaig belt. Other features: CT, Creag Mhor Thollaidh block; FF, Flowerdale fault; IB, lalltaig block; TF, Tollie Farm; SL, Shieldaig Lodge; SS, Sidhean Mor. LMG amphibolites shown by dashed ornament; metasediments, dotted; t, Torridonian. From Lei & Park (1993).

shear zones are locally affected by open or chevron-style F4 folds with steeply plunging axes (see Fig. 6.13). Typically, these have a sinistral asymmetry and are thought to be related to later sinistral movements on the shear zones. The Leth-chreige crush belt has undergone a sinistral displacement of at least 4 km, judging from the matching of L2 lineation plunge patterns across the belt. 6.6 Summary of Laxfordian tectonic history D1 structures comprise the first fabrics in the Scourie dykes and Loch Maree Group; however, they are only clearly distinguishable where later structures are weakly developed. They can be studied in the dykes of the Creag Mhor Thollaidh and Loch Braigh Horrisdale areas where they are unaffected by the main D2 and D3 shear zones. There, the amphibolite-facies fabrics occur in

narrow shear zones, typically with a sinistral, S-up shear sense which may be due to rotation in the less deformed footwall and hanging wall of the D2 shear zone (see Fig. 6.17A). These fabrics are folded by F2 folds at the margins of the main D2 shear zone on the NE limb of the Tollie antiform. D1 structures are rare in the metasedimentary schists of the LMG, but more common in the amphibolites within the Ard gneisses, where they comprise an early foliation seen in the fold hinges of tight F2 folds. D1 in the dykes does not necessarily correspond to DI in the LMG. The main deformation phase in the Gairloch area was D2; it also occurred under amphibolite-facies conditions and produced a strong planar and linear fabric both in the LMG and in the adjoining basement gneisses. When the effects of the large-scale D3 structures are removed, the D2 foliation appears as a relatively gently-inclined structure in the form of a sheer zone of the order of

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Fig. 6.15. Photomicrographs of mylonitic rocks; sections cut parallel to lineation and perpendicular to foliation. (A) Mylonitized banded-iron-formation showing sinistral S-C structure (D2), Flowerdale crush belt, [822 741]; width of field, 5mm (x 15). (B) Mylonitic gneiss, with partly recrystallized feldspar augen in fine-grained recrystallized matrix, showing sinistral S-C structure (D3), NE lalltaig crush belt, [811 733]; width of field 3mm (x20).

5 km in thickness. The L2 elongation lineations over most of the LMG outcrop, when rotated into their presumed D2 orientation, imply a WNW-ESE movement direction, consistent with the regionally developed high-grade Laxfordian D2 fabrics throughout the Lewisian (Coward & Park 1987). The main movement sense is considered to be top to WNW (Fig. 6.17A,C) although some structures imply the opposite movement sense. However, L2 over most of the Ard gneiss outcrop has a different orientation, implying a NNE-SSW movement direction before rotation. There is also very limited evidence of early NW-plunging elongation lineations within the LMG metasediments, which suggests the possibility that the NW-plunging lineations typical of the Ard gneisses may have been more widespread but are now overprinted by later SEplunging fabrics. The zircon age of 1.90 Ga for the emplacement of the Ard gneiss is regarded as the upper age limit of the D2 event. A lower limit is the c. 1.70 Ga age of the post-D2 pegmatites (see Section 5.3.3). In the D3 deformation, the D2 fabrics are refolded by upright NW-SE folds, and steep NW-SE dextral shear zones are developed (Fig. 6.17B,D), accompanied by widespread retrogression to greenschist facies. This event can be approximately dated at 1.70 Ga (see Section 6.1.2). The apparent change in the main movement sense from D2 to D3 implies a regional kinematic change from WNW-ESE convergence to N-S or NNE-SSW convergence. The D1 and D2 structures represent ductile deformation that took place under amphibolite facies at mid-crustal levels but, during D3, when the rocks were at a shallower crustal level, retrogressive high greenschist-facies assemblages were formed in the footwall of the Gairloch shear zone. The oblique-slip tectonic regime that characterized the D3 phase was probably replaced by a strike-slip regime during D4, which was responsible for the later movements on the NW-SE brittle-ductile crush belts. This would represent a significant change from previous regimes, which all involved a substantial dip-slip component. From the end of the D3 movements until the deposition of the Torridonian, the contemporary ground surface may have been within some 10 km of the present erosion level and further large vertical movements between blocks probably did not occur. The asymmetrical (sinistral) D4 folding that affects the catacla-

sites may be related to the 1.5km dextral displacement on the Flowerdale fault, which clearly postdates the cataclasites of the Creag Bhan shear zone (Fig. 6.17E). The shear direction for strikeslip motion on the Flowerdale fault is oriented 20-30° anticlockwise from the D3 shear direction. The reversal to a sinistral sense of movement on the crush belts may thus be attributable to a clockwise rotation of the Gairloch area under continued dextral shear, as a result of which the NW-trending zones could have been rotated into an orientation where they would have been subjected to both shortening and antithetic sinistral shear (see Lei & Park 1993, fig. 2). In the absence of reliable geochronological evidence, the later structures cannot be dated firmly; however, Moorbath & Park (1971) suggest that a concentration of K-Ar reset ages at between c. 1500 and c. 1400 Ma may date these D4 movements. Some of the later movements on the crush zones may be of Grenville age (c. 1.11.0 Ga; see Section 1.2.8).

6.7 Faults As shown on the geological map, the Lewisian rocks of the Gairloch district are affected by numerous faults, which can be delineated relatively easily because of the excellent quality of the exposure over most of the area. It can be seen from the Geological Survey 1 :50000 map (sheet 91) that areas of less good exposure (e.g. to the north and south of the area considered here) show fewer faults, thus the observed frequency of faults is partly a function of quality of outcrop. However the basement gneisses are generally more heavily faulted than in the LMG outcrop. There is also a tendency for faults to occur in linked groups: e.g. the N-trending set west of Loch Bad an Sgalaig. Many of the larger faults are associated with wide cataclastic zones, containing fault breccia veined by pseudotachylite (Park 1961). The later brittle movements on the crush zones described above (Section 6.5.3) imply that the latter belong to a special class of NW-trending strike-slip faults. For convenience, the faults are differentiated here into those of pre-Torridonian and post-Torridonian age. although there are numerous examples whose age cannot be determined directly.

LAXFORDIAN STRUCTURE AND METAMORPHISM

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Fig. 6.16. (A) Pseudotachylite vein in semipelite; width of field, 27mm (x5). (B) Relict spherulitic structure in pseudotachylite; width of field, 500 um (x250). A, B, from Flowerdale crush belt [823 742].

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Fig. 6.17. Interpretative summary of kinematic chronology in the Gairloch area (from Lei & Park 1993). A, D2: top-to-west shear on low-angle shear zone, showing rotation of dykes and sinistral shear sense in horizontal plane. Later reversal of movement (not shown) attributed to extensional collapse produced topto-east shear and dextral shear sense in horizontal. B, D3: dextral, north-up, shear on steep Gairloch shear zone (dotted ornament) resulted from N-S compression. Gently inclined shear zone is now steep due to D3 movements. C-F, diagrammatic maps of Gairloch showing crush belts (black), principal folds and faults (dashed), to indicate sense of movement on the main strucures from D2 to post-D4. Filled half-arrows indicate shear sense; open arrows indicate inferred movement directions. C, Top-to-west movement on shear zones (now steep) gave sinistral, south-up shear sense. D. D3 movements resulting from dextral transpression caused NW-trending folds and dextral N-up shear sense on steep shear zones. E. D4 movements caused sinistral strike-slip on steep shear zones, due possibly to clockwise (dextral) rotation of Gairloch area between main boundary faults. F, Post-D4 strike-slip faults form conjugate set attributed to overall north-south sinistral shear.

LAXFORDIAN STRUCTURE AND METAMORPHISM

6.7.1 Pre- Torridonian faults The Flower dale fault. The most prominent fault in the district is the WNW-trending Flowerdale fault, which crosses the centre of the map area, causing a 1.5 km dextral displacement of the vertical units of the LMG (see map, Fig. 6.17F). This fault is clearly overlain by Torridonian deposits at both ends. The fault contains a narrow zone of fault breccia and pseudotachylite. It is one of only three faults in the map area with this WNW trend. It postdates the cataclasites of the Creag Bhan shear zone and is cut by one of the N-trending set of faults. Lei & Park (1993) suggest that the movement on this fault may be related to the sinistral movements on the NW-trending crush zones (see above) and could be late Laxfordian in age. However a Grenville age is also possible. North-south thrusts. The Kerry road ore deposit, studied in detail by Jones et al. (1987) is bounded by two faults. Jones et al. showed that the faults are thrusts, with variable trends, the more easterly of which dips at 23° SE, and is also cut by one of the North-south faults. They note possible comparisons with the Moine and Outer Isles thrusts; the probably pre-Torridonian age would rule out a Caledonian age but a Grenville age is a possibility (see below). North-trending strike-slip faults. North-trending faults are relatively common, and occur throughout the area. The main north-trending faults that cross the LMG outcrop have a clear, dextral, strike-slip sense of movement. Three faults of this set cross the supracrustal belt in the central part of the area, 1 km west of Loch Bad an Sgalaig. The two eastern faults form a branching system that is itself linked to the western fault by two conjugate WSW-trending sinistral faults. The north-trending faults either terminate against or displace the Flowerdale fault, and hence post-date it. At their southern extremities, at Dubh Loch, these faults are cut by a NEtrending, post-Torridonian, normal fault and strike-slip displacement does not continue into the Torridonian cover on the SE side of Dubh Loch. They are thus regarded as pre-Torridonian. NE-trending normal faults. These are the most abundant type of fault, found over the whole area, but particularly in the basement gneisses. They are approximately perpendicular to the main structural trend, and are parallel to a widely distributed set of extensional joints that have a marked influence on the topography.

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Several of these faults appear to cross the main Flowerdale fault without displacement (e.g. around Lochan Dubh nam Biast, at [862 732] and some also end against members of the north-trending set, for example at [816 743] and [866 766], and are presumed to postdate both these. The extensional joints were probably initiated in a phase of late Laxfordian, orogen-parallel extension. However, many of the NE-trending faults may well be related to the extensional rifting which accompanied both the Stoer and Torridon Group deposition (see Stewart 1991; Beacom et al. 1999). This interpretation is strengthened by the presence of numerous sandstone dykes with northeasterly trends parallel to this fault set. Although many of the northeasterly faults displace the Torridonian deposits (see below), the post-Torridonian movements probably re-used pre-existing faults.

6.7.2 Post-Torridonian faults The Loch Maree fault. The northeastern boundary of the area is formed by the Loch Maree fault and its splay, the Tollie Farm fault. The Loch Maree fault is parallel to the major NW-trending, brittle-ductile shear zones described above, and is probably coeval with them. However, there is also a large post-Torridonian displacement (c. 1500m minimum downthrow to the NE at Poolewe) which is reflected on the 1 :50 000 mapsheet, and a smaller displacement affects the outcrops of the Cambrian sequence and the Moine Thrust belt at the SE end of Loch Maree (see BGS sheet 92). NE-trending normal faults. Some of the abundant northeasterly faults described above cross-cut the Torridonian cover. Several of these faults form the boundaries of the Torridonian inliers within the Gairloch map area and have measurable throws of the order of several tens of metres. These include the fault running through Shieldaig Farm [808 723] and the fault running parallel to the road, 300m to the east. Another significant northeasterly fault bounds the Lewisian outcrop SW of Dubh Loch, and displaces the base of the Torridonian by at least 50m. These northeasterly faults are part of a widely distributed set of sub-parallel normal faults found throughout the British Isles and adjoining continental shelf whose later movements were associated with Mesozoic to early Tertiary NW-SE extension (e.g. Roberts & Holdsworth 1999).

Chapter 7 Tectonic Interpretation and Regional Significance

7.1 The Archaean (Scourian) The fragmentary nature of the outcrop of least modified Scourian rocks limits any attempt at interpretation of their tectonic history. However, the three main areas where such rocks can be studied (Loch Braigh Horrisdale, Creag Mhor Thollaidh and Ard lalltaig) all show certain common features. The rocks are all orthogneisses, dominantly granodioritic in the SW (Braigh Horrisdale), but quartz-dioritic to dioritic with abundant mafic inclusions at Creag Mhor Thollaidh and Ard lalltaig. Structurally, there is considerable uniformity; the early gneissose banding/foliation is generally steeply dipping with a NEerly to northerly trend. This structure (S2) has been produced by the D2 deformation of the igneous protoliths of the gneisses, which also locally contain a previous foliation, S1. A similar structural pattern is found to the south of the Gairloch area, at Ruadh Mheallan in the Torridon district, and also to the north, at Gruinard Bay. Thus, all the areas throughout the Southern Region of the Lewisian complex where the Scourian structure can be recognized in a relatively unmodified state, involving an outcrop width of around 35km, appear to present a common structural style. This has obvious implications for any collisional model for the Palaeoproterozoic, suggesting that the Archaean blocks on either side of the Loch Maree Group outcrop may have had a common origin, or may originally have been contiguous (see Section 7.3). The intense deformation, gently-dipping foliation and high pressure granulite-facies metamorphism that characterize the Scourian of the type area in Scourie and Assynt are not matched in the Southern Region. The tectonic model of Park & Tarney (1987) for the Scourian of the type area envisages these rocks being formed at a low-angle subduction zone affected by progressive underplating and generating voluminous tonalitic melts (Section 1.2.3). The Southern Region, however, including the Gairloch area, may represent an originally higher crustal level where the deformation induced by the underplating process was weaker or even absent. The steeply-dipping NEerly trending foliation may be the result of deformation post-dating the subduction/accretion episode, or it may represent an upwards steepening and weakening of the deformation brought about by the accretionary process. The lack of modern precise geochronology in this area is a serious drawback to tectonic interpretation. The only relevant information comes from Gruinard Bay, where Corfu et al. (1998) linked the steeply-dipping NE-trending fabric to an episode of hornblende-granulite facies metamorphism dated, by U-Pb zircon, at 2730 Ma. Corfu et al. (1998) also gave evidence for an earlier event, between 2790 and 2850 Ma, which may have been related to the emplacement of the tonalitic and gabbroic rocks. In view of the general lithological and structural similarity between the Gruinard and Gairloch areas, and the comparatively short distance between them, these dates may also apply to the early Scourian gneisses of the Gairloch area. There is a problem over the correlation of structures and events between the Central and Southern Regions of the Lewisian. The most recent geochronological work suggests that the Badcallian event sensu stricto in the type area of Badcall and Scourie may be considerably younger than formerly thought (at c. 2.5 Ga) (see Section 1.2.4) and does not appear to have a counterpart in the Southern Region, and the c. 2.8 Ga intrusive event in the Gruinard area, referred to above, does not appear to be matched in the Central Region, where the protoliths of the gneisses are dated at c. 2.9-3.0 Ga. It is not yet possible to determine the relative positions of the Archaean blocks of the North Atlantic area by palaeomagnetic

means. However, rocks and dated events similar to those of the Southern Region of the Lewisian are widespread in the Archaean regions that surrounded the Lewisian during the Palaeoproterozoic (see Fig. 6.1), and it is widely believed that at the end of the Archaean, all these regions probably formed part of a larger continent that subsequently broke up during the early Palaeoproterozoic.

7.2 The Inverian: earlier Palaeoproterozoic? Structures that have been assigned to the Inverian (as defined in Section 2.3) can only be studied easily in the SW and NE of the Gairloch area on either side of the broad central belt of Laxfordian deformation, and in the lalltaig block. They are particularly well developed immediately NE of the Braigh Horrisdale river in the south, and in the Creag Mhor Thollaidh area in the north. In both these areas, the Scourian foliation is affected by open to tight folds with steeply dipping, NW-trending axial planes and by an associated axial planar foliation. This Inverian fabric, developed under amphibolite-facies conditions, resulted in pervasive recrystallization of the Archaean gneisses. From the appearance of the pre-Laxfordian gneissose banding in the areas of low-strain Laxfordian deformation, it is inferred that the belt of Inverian deformation involved all the Archaean gneisses throughout the central highly deformed belt in the Gairloch area. This belt of Inverian deformation was interpreted by Park et al. (1987) as part of a much broader ductile shear zone, the northeastern boundary of which can be recognized near the Gruinard River (Fig. 1.3; Crane 1978). The lineations associated with this shear zone can be recorded where they are unmodified by Laxfordian deformation in the weakly deformed outer parts of the shear zone. There, they are the result of the intersection of the Scourian banding and the steeply-dipping Inverian foliation and do not indicate the shear direction. From the geometry of these outer regions, the shear zone appears to be strike-slip with a subhorizontal shear direction. From the anticlockwise sense of rotation of Inverian fabrics as the zone is approached from the south, in the Braigh Horrisdale area, the shear sense appears to be sinistral (see Section 2.4.1). However this relationship may have been modified by the subsequent Laxfordian deformation and should not be regarded as diagnostic. Major shear zones of earliest Palaeoproterozoic age have been recorded in several of the Precambrian blocks of the North Atlantic region, and may perhaps be associated with the movements that accompanied the break-up of the Archaean supercontinent. The lack of reliable palaeomagnetic data, referred to above, makes it impossible to reconstruct the geometry of the region prior to the Palaeoproterozoic collisions of c. 1.9 Ga onwards. However, the Inverian structure of the Southern Region appears to form a continuum; i.e. the basement on either side of the Loch Maree Group outcrop (both at Gairloch and Loch Maree) appears to belong to the same early Palaeoproterozoic crustal block.

7.3 The Scourie dyke swarm: events from c. 2.4 to c. 2.0 Ga? Although the 'Scourie dyke' swarm in the Gairloch area is as yet undated, it has traditionally been assumed to correspond to the main dated suite in the type areas of Scourie and Assynt, considered to be c. 2.4 Ga in age, and to have been emplaced either shortly after, or indeed during, the Inverian event (see Section 1.2.6). If the dyke swarm in the Southern Region proves to be of a different age to the type Scourie dyke swarm, this will

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Fig. 7.1. Cartoon profiles illustrating the possible tectonic evolution of the Gairloch area during the early Laxfordian. from Park et al. (2001). (A) Stage 1: a thick sequence of plateau basalts, accompanying oceanic sediments and island-arc material, is scraped off the lower plate, tectonically mixed with turbiditic clastic sediments derived from the upper plate, and underplated beneath the continental crust of the upper plate. (B) Stage 2: lower-plate continental crust has underthrust the upper plate and imbricate slices of lower-plate material have been emplaced beneath the previously underplated oceanic material. (C) Stage 3: extensional collapse of the overthickened Stage 2 crust.

require revision of those parts of the structural chronology based upon it. The remarkable concentration of dykes in the Gairloch area, and the evidence for structural control by the pre-existing Inverian structures (see Section 3.1) suggests that the two phenomena are genetically linked, and that the basement in the Gairloch area may mark the site of a failed rift in earliest Palaeoproterozoic times. The evidence from the metavolcanics and associated marine sediments of the Loch Maree Group (discussed below) indicates the presence of at least a small ocean basin at the site of the LMG. This ocean probably existed at around 2.0 Ga, that is, about the same time as the younger suite of Scourie dykes was emplaced in the Central Region. The emplacement of the younger set of dykes (which may or may not have representatives in the Gairloch area) may therefore have heralded the break-up of the 'Lewisian' sector of an early Palaeoproterozoic continent. The apparent continuity of Archaean and Inverian structures throughout the Southern Region suggests that the whole region belongs to one side of the LMG ocean, and that the LMG at Gairloch has been structurally interleaved with this continental basement.

7.4 The Laxfordian: later Palaeoproterozoic The radical revision of the Palaeoproterozoic tectonic interpretation of the Gairloch area is based partly on new precise zircon dating and partly on a reinterpretation of the origin and significance of the Loch Maree Group (Park et al. 2001). The revised views also benefit from a new, more accurate, palaeomagnetic reconstruction of the Baltica-Laurentia fit during the Palaeoproterozoic (Buchan et al. 2000). The following account is based largely on Park et al. (2001).

7.4.1 The early Laxfordian: model

sitbduction-accretion-collision

Park et al. (2001) discussed the possible tectonic processes and history that led to the intercalation of the plateau-arc sequence, the active continental margin sediments, and the magmatic arc rocks of the Ard gneiss unit. They concluded that the LMG represents a deformed accretionary complex in which the various units of the LMG have been progressively accreted onto the upper plate of a subduction zone (Fig. 7.1). The LMG metavolcanics are interpreted as oceanic plateau basalts and the associated sediments as a shallow marine assemblage. The semipelites of the LMG represent the immature clastic sediments of an accretionary prism, incorporating both juvenile and older basement source materials. The interleaving of the semipelites with the oceanic assemblage probably reflects the accretion of separate tectonic slices. The granodiorite to tonalite protolith of the 1.90 Ga Ard gneiss, interbanded with the LMG along its SW margin, may have been generated by melting of subducted mafic rocks and intruded into the overlying LMG. The accretionary process was then followed by collision with a continental lower plate. A possible high level analogue lies at the continental margin of Colombia where an 87 Ma Caribbean oceanic plateau assemblage is intercalated with significant volumes of highly foliated and lineated shale-greywacke (flysch) (Kerr et al. 1997a.b). Kerr et al. suggest that the outboard plateau rocks of the Columbian example were accreted and deformed during late Cretaceous to early Tertiary subduction to an arc-influenced, continental margin sedimentary sequence. In this case, it is mainly the upper-level altered basalts that were accreted, whereas the underlying gabbros and mantle components were apparently consumed in the subduction zone (Kerr et al. 1997a.b).

THE LEWISIAN GEOLOGY OF GAIRLOCH, NW SCOTLAND

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Fig. 7.2. Schematic cross-section through the Southern Region of the mainland Lewisian, showing the principal structural elements, and the relationship between the Loch Maree Group and the basement Scourian gneisses resulting from the later Laxfordian deformation. The originally gently inclined D2 structure, including the early Laxfordian shear zones, has been rotated into a steep attitude by the later Laxfordian (D 3 ) upright folds (e.g. the Tollie and Carnmore antiforms) and by the Gairloch shear zone. The complementary synform to the Tollie antiform (the Gairloch synform) is faulted out by the Creag Bhan crush belt. After Park et al. (2001).

A deeper level, and thus perhaps more appropriate, analogue may lie in the Tertiary crystalline massifs in the Alpine orogen of northern Greece (Barr et al. 1999). Although similar in many respects to the Colombian example, there is a much higher proportion of Tethyan limestones in Greece. During the accretionary process these were converted to coarse marbles, and are now associated with blueschists and eclogites (many now retrogressed). These components of the subduction complex must have been taken down to mantle depths before eventual exhumation 2030 million years later, a process accompanied by widespread migmatization. Locally there are banded ironstones and base metal mineralization, presumably originating from the ocean floor. The relative abundance of rock types and specific structural attributes may differ between the Gairloch area and the suggested Columbian and Greek analogues, but the first-order similarities are striking and appear to support a common tectonic setting and history. The differences are not surprising when considering the many variables of subduction-accretion and growth of accretionary wedges, many of which are only now becoming better documented and understood (von Huene & Scholl 1991, 1993; Taira et al. 1997; Barr et al. 1999). Park et al. (2001) noted that a possible factor influencing the accretionary process is the existence of large rivers issuing at active continental margins. The weight of sediment formed by such rivers loads and depresses the downgoing plate and flattens it, increasing the contact between the upper and lower plates, and sediment is scraped-off against the continent, becoming severely deformed. Moreover, the lithified material from the ocean crust can be involved in this process and scraped-off into the developing accretionary prism (Kimura & Ludden 1995). Another factor is the nature of the subducting oceanic slab. Normal ocean floor is cold and dense, and provides most of the 'slab-puir force, normally subducting completely. However, the exotic components of the oceans tend to get scraped off during shallow subduction: i.e. island arcs, seamounts, aseismic ridges and ocean plateaus, together with their carapaces of carbonate banks and silicic chert. Oceanic plateaus can be fairly large (e.g. the size of France) and were formed over a relatively short time-span; they are normally too warm and buoyant to subduct immediately (Cloos 1993) and therefore have a tendency to promote subduction-flip or back-step of the subduction zone (Saunders et al. 1996). In summary, the later Palaeoproterozoic history of the Gairloch

area is interpreted as the result of the lateral accretion of oceanic plateaus and primitive island arcs. This involved severe tectonic imbrication and mixing of oceanic, volcanic arc and continentally derived components followed by continental collision. The premetamorphic tectonic breccia within the Flowerdale marble belt (see Section 4.6.1) is thought to represent one of the early decollement zones produced during this accretion process. The D2 deformation is interpreted as the result of a major, lowangle shear zone with a westerly directed sense of movement (see Fig. 6.17A). This movement would correspond to eastwards underthrusting during the accretion or collision process. The D2 structures with the SE-plunging lineations that affect both the LMG and the adjoining gneisses on both sides, i.e. the gneisses of the lower continental plate, must therefore, at least partly, relate to the collision process. However the Ard gneiss units with the steep NW-plunging D2 lineations give an indication of a different movement direction that may relate to the accretionary stage, but which may have been largely obliterated in the LMG metasediments. It is probable that the D1 foliation in the LMG was also formed during the accretion stage rather than during collision, in which case the D1 structures in the LMG would not correspond to those in the Scourie dyke swarm. A possible sequence of events is represented in cartoon form in Figure 7.1. Stage 1 (A) shows the original accretionary complex and is followed by Stage 2 (B) where the subduction complex becomes sandwiched by thrusting between two Archaean continental blocks. The D 1 /D 2 deformation may reflect a possibly continuous deformation from the accretionary through to the collision stage. Stage 3 (C) represents the later D2 extensional phase resulting from orogenic over-thickening of the crust during Stage 2. The whole of the Southern Region of the mainland Lewisian is thought (Section 7.3) to have formed part of the same continental block during the Palaeoproterozoic, and this block corresponds to the lower plate of the collision zone shown on Figure 7.1. The upper continental block does not appear to be represented in the Gairloch area but could be represented by the Central Region, with the suture between the two plates being concealed by Torridonian cover somewhere around Loch Broom (i.e. at the right hand end of the section shown on Fig. 7.2). The age of formation of the LMG is between 2.0 and 1.90 Ga from the age of the youngest sediment source ages, and the accretionary process can be dated precisely by the 1.90 Ga Ard

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Fig. 7.3. Reconstruction of Laurentia and Baltica during the Palaeoproterozoic. Note that the Lewisian lies within a continuous Palaeoproterozoic belt extending from the Torngat belt of Laurentia through the Nagssugtoqidian of Greenland to the Lapland-Kola belt of Baltica. The arrows show the inferred movement directions. NAC North Atlantic craton; Nag, Nagssugtoqidian belt. From Buchan el al. (2000).

gneiss. The date of collision (D2) is not known accurately but may be around 1.87 Ga by analogy with comparable events in South Harris (see Section 6.1.2).

is oblique dextral, with a NE-up component; i.e. the movement is dextral transpressional (see Fig. 6.17D). implying that the Gairloch area was subjected to approximately N-S compression during this episode.

7.4.2 Later Laxfordian structural history The generally gently inclined planar structures formed during the D2 event were rotated into their present steeper attitudes during the D3 event in much of the Gairloch area. The main D3 structures involved in this process are the NW-trending Tollie antiform and Gairloch synform, and these structures are matched on the NE side of Loch Maree by the Letterewe synform and Carnmore antiform (Fig. 7.2). The SW limb of the Tollie antiform and NE limb of the Gairloch synform occupy the 6 km wide, NW-trending Gairloch shear zone. Movements both on this shear zone and on the younger Creag Bhan crush belt have obscured the hinge zone of the Gairloch synform. These main D3 structures are accompanied by locally developed minor folds, in places with an S3 axial planar foliation, and by widespread partial retrogression of the D2 amphibolite-facies mineral assemblage to greenschist facies. Numerous granitoid pegmatite bodies and, less commonly, microgranite dykes have invaded the area during the late Laxfordian period. These bodies are particularly common around the hinge zone of the Tollie antiform, where they clearly cross-cut the D2 structures and are sheared and retrogressed by D3, thus forming a useful chronological marker (cf. Holland & Lambert 1995). A zircon age of 1694 Ma (Park et al. 2001) is interpreted as the emplacement age of the pegmatites. This date gives an upper limit to the D3 phase which, from other evidence, is thought to have occurred around 1.7 Ga (see Sections 1.2.8, 6.1). The sense of movement on the main Gairloch shear zone and on the many small-scale D3 shear zones throughout the Gairloch area

7.4.3 Post-D3 tectonic historv During D4, widely distributed, steeply plunging folds were formed, typically associated with narrow cataclastic zones and with localized retrogression to zeolite-facies assemblages. These structures are associated with sinistral strike-slip movements on several narrow, steep, NW-trending. brittle-ductile shear zones that disrupted the continuity of the D3 geometry (see Fig. 6.17). These D4 structures may have been caused by a clockwise rotation of the Gairloch area under an overall dextral strike-slip regime (see Section 6.6). Faults are divided into a pre-Torridonian set, comprising mainly north- and NW-trending dextral, strike-slip faults and a postTorridonian set, comprising mainly NEerly-trending normal faults. Although many of the latter set of faults affect the Torridonian beds, they are likely to have been initiated by late Laxfordian orogen-parallel extension and to have been re-activated by extension accompanying Torridonian deposition and also during subsequent extensional episodes. 7.5 Palaeoproterozoic belts of the North Atlantic region The Lewisian complex of NW Scotland formed part of a widespread Palaeoproterozoic system of orogenic belts throughout the North Atlantic region. A recent palaeomagnetic reconstruction (Buchan et al. 2000; Fig. 7.3) of Laurentia and Baltica during the late Palaeoproterozoic to early Mesoproterozoic shows a close geometric relationship between the Nagssugtoqidian belt of

THE LEWISIAN GEOLOGY OF GAIRLOCH, NW SCOTLAND

Greenland and the Lapland-Kola belt of Baltica within the reconstructed continent. The Nagssugtoqidian belt of East and West Greenland appears to continue through the Lewisian complex of NW Scotland, a correlation also suggested by several previous workers (e.g. Myers 1987). Comparisons of the tectonic histories of the Nagssugtoqidian and Lapland-Kola belts have been made by Bridgwater et al (1990), Kalsbeek et al. (1993), and Park (1994, 1995). Park (1994) noted that the kinematics associated with the peak temperatures in the two belts correspond best when the belts are approximately perpendicular as shown in Figure 7.3. There is a general consensus that the Nagssugtoqidian and Lapland-Kola belts, together with the Torngat and New Quebec orogens of Laurentia, are broadly coeval and have resulted from the progressive accretion of a number of terranes during the period 1.901.83 Ga (e.g. see references in Park 1995). While most of the orogenic belts show evidence of a continental-margin setting, including subduction and sedimentation, and eventual continentcontinent collision, a similar history and setting had not, until recently, been proposed for the Lewisian. This mainly reflected an apparent lack of magmatic arc rocks and Palaeoproterozoic continental margin sediments, and the apparent similarity of the Archaean basement on either side of the Palaeoproterozoic LMG belt. Calcalkaline plutonic suites ranging from diorites through tonalites to granodiorites form a volumetrically small, but significant, suite in the Palaeoproterozoic continent-continent collision belts of the North Atlantic region, emplaced into both older sialic crust and Paleoproterozoic sediments. Emplacement

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ages range between 1.85 and 1.95 Ga (e.g. Kalsbeek et al. 1987, 1993; Kalsbeek & Nutman 1996; Scott 1995). The 1.90 Ga magmatic age for the Ard gneisses, originally thought to be part of the Archaean basement, (Section 5.1.4) indicates the existence of a previously unknown Palaeoproterozoic magmatic arc within the Lewisian sector of the NagssugtoqidianLapland-Kola collisional belt. Together with the other evidence from the Loch Maree Group of an active margin setting, this places the Lewisian complex firmly within the Palaeoproterozoic collisional belt system of the North Atlantic region. The early Laxfordian (D2) convergence direction in the Gairloch area is consistent with kinematic indicators in the Lapland-Kola, Nagssugtoqidian and Torngat collisional belts (Fig. 7.3) which suggests that a common movement pattern prevailed during the Palaeoproterozoic amalgamation of the North Atlantic supercontinent. The Archaean cratons and Palaeoproterozoic collisional belts of the North Atlantic region had become welded together into a large continental mass by around 1.84 Ga, after which an active margin was developed on its SW side (with respect to the present orientation of Scotland) (Fig. 7.3). It is considered (Park 1994, 1995) that the late Laxfordian events in the Lewisian, including the emplacement of the granitoid pegmatite suite and the D 3 deformation in the Gairloch area, reflect the influence of events taking place at this active margin, which, in Scotland is represented by the Rhinns complex of Islay. The Rhinns complex forms part of the Labradorian-Gothian belt of SE Canada and SW Scandinavia which was active in the period 1.85-1.50 Ga (see Park 1995 and references therein).

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Index

accretion/collision 3, 7, 51, 68-70, 71 Achiltibuie 5 actinolite 40, 43 agmatitic gneiss 12 Airigh na Cloiche 48 albite 45, 58 almandine-amphibolite fades 51 aluminium 16, 35, 51 Am Feur-loch 13, 33, 36 amphibolite-facies metamorphism 3-7, 16, 18-19, 23, 51, 52 amphibolites in Ard gneiss 38, 39, 45, 56, 61 dykes 3, 21-27 Loch Maree Group 38-42, 44 Scourian/Inverian 12-13, 14, 18 sheets 3, 7, 21, 29, 45 An Ard 45 An Dun 45 An Groban 39 andesine 39 ankerite 43 anorthosite 5 apatite 12, 24, 29, 31, 35, 36, 39, 45, 47 aplite 47 Archaean basement gneisses 3, 11-19, 29, 44, 51, 62, 67, 71 Ard gneiss 3, 7, 45-47, 71 amphibolite sheets in 38, 39, 45, 56, 61 and Laxfordian 7, 45, 51, 52, 56, 61, 62, 68-70 meta-limestones in 33 schistose semipelite bands in 29, 31 Ard lalltaig 12, 14, 17, 18, 58, 67 Assynt 6, 16, 25, 67 Auchtercairn 36 Aundrary basite (amphibolite) 29, 36, 38-42, 43, 58 Badcallian 3, 6, 8, 67 Baltica 1, 68, 70-71 banded-iron-formation (BIF) 7, 32, 33, 34-35, 43, 44 and Laxfordian 51, 52, 54 banding and foliation, Scourian/Inverian 6, 13, 16-17, 18, 22, 23, 54-55, 67, see also foliation basaltic origin 14, 41-42, 44 and hydrothermal alteration 37-38, 41-42, 44 biogenic origin 38, 44 biotite in Archaean gneisses 8, 11-12, 13 in Ard gneisses 45, 46, 47 in granite sheets 47 in LMG 29, 31, 35, 39, 43, 52, 58 in Scourie dykes 24, 25 biotite-muscovite semipelites 5 bornite 43 brecciation 36, 51, 52-54, 59-60, 62, 65, 69 bronzite-picrite dykes 7 Buainichean gneiss 29 calcareous chloritic schist 52 calcic plagioclase 16, 18, 39, 51 calcite, in LMG 29, 33, 34, 35, 39, 43, 45 Canisp shear zone 6, 7 carbonates, Loch Maree Group 32, 33-34, 35, 38, 44 Carnmore 7 Carnmore antiform 47, 57, 70 cataclastic rocks 35, 51, 52, 58-62, 70, see also brecciation

Central Region 3, 6, 7-8, 9 Scourian gneisses 4-6, 67 Scourie dykes 7, 25-27, 68 chalcocite 43 chalcopyrite 42, 43 chamosite 35 Charlestown schist 29-32, 35, 38, 40, 46, 56 chert 35 chlorite in Archaean gneisses 12 in Ard gneisses 45, 46 green 29, 34, 45 in LMG 29, 33, 34, 35, 36, 39, 40, 43, 58 chloritic schists 35-36, 38, 43, 52 chronology see geochronology; structural chronology clinopyroxene 5, 18, 23, 24 Cloiche marble belt 29, 34, 36, 52 amphibolites in 36, 38 gneisses in 29, 36, 45, 56, 57 sulphide deposits in 43 Coille Airidh na Cloiche 33, 36 collisional origin see accretion/collision copper-bearing sulphide deposits 42-43 Creag Bhan crush belt 29, 33, 34, 36, 58, 60, 62, 70 Creag Mheall Beag 52 Creag Mhor Thollaidh and Laxfordian 55-56, 60, 61 Scourian and Inverian structures 17-18, 55 Scourie dykes 21, 22, 25, 47, 51 Creag Mhor Thollaidh gneisses 12, 13, 47, 67 Creag Mhor Thollaidh granite sheets 47 crustal history 6-7, 14, 27, 31-32, 44, 67-9, 71 diopside 33 diorite 6, 13, 14, 67, 71 dolomitic marble 33-34 Druim na Fearna 43 Druim Ruadh 45 Drumbeg 5 Dubh Loch 29, 29, 45, 65 dunite 5 'early basic' amphibolites 5, 12-13 'epidiorites' 24-25 epidote in Archaean gneisses 12, 13, 18 in Laxfordian intrusives 45, 46, 47 in LMG 24, 29, 36, 51 in Scourie dykes 24 Tollie antiform 58 Eu anomalies 14, 31 exhalative origin 38, 43, 44 faults 17, 29, 39, 62-5, 70 Flowerdale 33, 35 Flowerdale crush belt 52-54, 58, 60 Flowerdale fault 29, 39, 62, 65 Flowerdale House 29 Flowerdale Mains 30 Flowerdale marble belt 29, 33-34, 35-36, 52 Flowerdale schist 29-32, 38, 40, 44 Flowerdale valley 34, 40, 52-54 flysch 68

78

folding Ard gneisses and amphibolites 45, 56 Creag Mhor Thollaidh 17-18, 55-56 Flowerdale valley 52-54 late Laxfordian 57-61, 70 Loch Braigh Horrisdale 16-17, 22 Mill na Claise fold 33, 39, 45, 52, 57, 58 foliation Ard gneisses 45 Loch Maree Group 29, 38-40, 52, 58 see also banding and foliation fuchsite 35 gabbroic rocks 14, 23-24, 68 Gairloch Hotel 29 Gairloch Pier 45, 47 Gairloch shear zone 8, 23, 51, 56-57, 58, 60, 62, 70 Gairloch supracrustal belt 17, 52-54, 60 Gairloch synform 58, 70 Gairloch-Kinlochewe Road 43, 52 galena 43 garnet-biotite schist 31, 36-38 garnet-grunerite schist 34 garnet in lalltaig gneiss 14, 18 in LMG29, 31, 34, 35, 43, 52 in Scourie dykes 23, 24 geochemistry amphibolites 14, 40-41 banded-iron-formation (BIF) 34-35 garnet-biotite schist 36-38 Laxfordian intrusives 47, 48 LMG carbonates 34 quartz-chlorite schists 36 quartzo-feldspathic gneisses 13-14 Scourie dykes 25-27 semipelites 31-32 geochronology 3 Inverian 6-7 Laxfordian intrusives 47, 48-49, 51, 62, 70 Loch Maree Group 7-8, 31-32, 44 Scourie dykes 5, 7, 9, 14, 67 goethite 35 gold 42, 43 gossans 43 granites 7-8, 47 granitic migmatite 7 granitoid orthogneisses 3 granitoid pegmatites 47-49, 51, 62, 70, 71 granodioritic gneisses 4, 7, 11-12, 13, 14, 67, 68, 71 granulite-facies metamorphism 5, 6, 16, 18 graphitic pelites 7, 32, 33, 35, 38, 44 greenalite 35 Greenland 1, 70-71 greenschist 8, 18, 45, 51, 56, 57, 60, 62, 70 Grenville-age movements 8, 62, 65 greywacke-type sediments 31, 36, 38, 44, 68 Gruinard Bay 4, 6-7, 16, 52, 67 Gruinard Bay amphibolites 14 Gruinard Bay gneisses 13-14 Gruinard River 56, 67 grunerite 34, 35, 54 haematite 24, 39 harzburgitic mantle material 27 hornblende gneisses 12, 14 hornblende schists 35, 38-42, 43 hornblende 5, 8, 12, 16, 18 in banded-iron-formation 35

INDEX

in Laxford intrusives 47 in LMG 36, 38, 39, 40, 52 in Scourie dykes 23, 24, 25 'hornblendite-metagabbro suite* 14 hydrothermal alteration 37-38, 41-42, 44 lalltaig see Ard lalltaig ilmenite 24, 39 intraplate setting 1, 42 Inverian 3, 4, 6-7, 16-18, 23, 51, 67 nomenclature 8-9, 16 and Scourie dykes 7, 16. 21-22, 23. 67-68 iron see banded-iron-formation (BIF); pyrite; sulphide deposits island-arc systems 42, 44, 69 Kerry Falls 35 Kerry Road ore deposit 43, 65 Kerrysdale 29, 40 Kerrysdale basite (amphibolite) 29, 33, 35, 36, 38-42. 54 sulphide deposits in 42-3 Kerrysdale schist 29, 33, 35 Langavat shear zone, South Harris 8 Lapland-Kola belt 1, 70-71 large-ion-lithophile (LIL) elements 27. 31, 41 Laurentia 1, 68, 70-71 Laxford shear zone 6 Laxfordian 3, 6, 7-8, 14, 19, 51-65 dating 7-8, 51, 68 early 7-8, 16, 45, 47, 51-57, 61-62, 68-70, 71 intrusive rocks 45-49 late 8, 16, 18,45, 57-58, 62, 70. 71 nomenclature 8-9 and Scourie dykes 21, 22, 23. 25, 51 k Laxfordianized gneisses 7-8 Leth-chreige crush belt 12, 22. 55. 58, 60. 61 Letterewe synform 57, 70 lineation 17-18, 23, 38, 45, see also foliation LMG see Loch Maree Group Loch Arich Mhic Criadh 25 Loch Bad a'Creamh 58 Loch Bad an Sgalaig 29, 33, 38, 62, 65 Loch Braigh Horrisdale 12, 16-17. 21-22, 25. 52. 61, 67 Loch Broom 69 Loch nam Buainichean 24, 58 Loch Druim na Fearna 33 Loch Kerry 38. 39. 45 Loch Laraig 12 Loch Laxford 6, 8 Loch Maree 12, 16, 22, 23, 51, 51, 65. 70 Loch Maree Fault 65 Loch Maree Group (LMG) 3, 4, 7, 29-44, 48 and Laxfordian 32, 36, 51, 56, 61-62. 65. 68-70, 71 Loch Tollie 47, 49 Lochan Dubh nam Biast 65 Lochan Dubh nan Cailleach 45 magmatic arc 3, 44, 68, 71 magnetite 24, 34, 35, 39, 43, see also banded-iron-formation 'main amphibolite suite' 14 malachite 43 mantle material 27, 44, 68 mapping history of study area 3 marbles, Loch Maree Group 32, 33-34. 38. 44. 51 marcasite 43 Meall an Doirean 12, 47 Meall an Spardain 47 Meall Aundrary 39 Meall Fuaraidh 39

INDEX

Meall na h'Inginn 40 meta-dolostones and meta-limestones 33-34 metabasalts 42, 44 metabasic ('early basic') amphibolites 5, 12-13 metadiorite gneisses 17, 23 metadolerite 25 metagabbroic gneisses 12, 14 metamorphic events 6, 7-8;, see also 'Badcallian'; 'Inverian'; Laxfordian; Scourian metasediments 3, 4, 5, 7, 29-35, 38, 44 micas 11-12, 29, 33, 35, 52, see also muscovite microcline 12, 45, 47, 49, 58 microgranite 47, 70 mid-ocean ridge basalts (MORB) 14, 42 migmatite 7 Mill na Claise basite (amphibolite) 29, 33, 35, 36, 38-42 Mill na Claise crush belt 58 Mill na Claise fold 33, 39, 45, 52, 57, 58 Mill na Claise gneiss 29, 45, 46, 58 Mullach nan Cadhaichean 12, 25 muscovite 5, 12 Laxforian intrusives 45, 46, 47, 49, 58 Loch Maree Group 29, 31, 35, 36 mylonites and protomylonites 18, 35-36, 47, 51, 52-54, 57, 59-60 myrmekitic texture 58 Nagssugtoqidian belt 1, 70-71 Nb anomaly 27, 41, 47 New Quebec orogen 1, 71 nomenclature 3, 7, 8-9, 14, 16 norite dykes 7 North Atlantic region 1-3, 70-71 Northern Region 3, 7-8 oceanic origin 6, 14, 27, 42, 44, 68-69 oligoclase 12, 29 olivine 12 olivine-gabbro dykes 7 olivine-tholeiitic amphibolites 14 orthopyroxene 5, 18 Outer Hebrides 4, 8 PAAS see post-Archaean argillaceous sediments palaeomagnetic reconstruction, Baltica-Laurentia 1, 68, 70-71 Palaeoproterozoic belts, North Atlantic 1-3, 70-71 pegmatites Badcallian 6 Laxfordian 47-49, 51, 62, 70, 71 Scourian 5, 16, 18 pelitic metasediments see graphitic pelites peridotite 5 perthite 5 petrography granodioritic gneisses 4, 11-12 Laxfordian intrusives 45—48 LMG amphibolites 38-40 Scourie dyke swarm 23-25 tonalitic gneisses 11-12 phlogopite 27, 34 phyllosilicates 31 picrite dykes, Assynt 25 plagioclase in Laxford intrusives 45, 46, 47, 49 in LMG 29, 31, 36, 39,40, 52 in microgranite sheets 47 in Scourian 13, 18 in Scourie dykes 23, 24, 25 Poolewe 65 post-Archaean argillaceous sediments (PAAS) 31-32, 44

79

pseudotachylite 51, 58, 59-60, 62, 65 pyrite 29, 35, 39, 42, 43 pyrophyllite 58, 60 pyroxene 5, 12, 18 pyrrhotite 42, 43 quartz-carbonate schist 42-43 quartz-chlorite schist 35-36 quartz-dolerite dykes 7 quartz-perthite pegmatites 5 quartz-plagioclase-biotite semipelitic schist 29-32 quartzites 34, 35, 36, 52 quartzo-feldspathic gneisses 5, 11-14, 17 Raasay 4 rare earth element (REE) analyses 6, 14 amphibolites (LMG) 41, 42, 44 Ard gneisses 47 garnet-biotite schist 36-37 Scourie dykes 26, 27, 42 semipelites (LMG) 31-32 Rhinns complex, Islay 71 ripidolite 35 Rona 4 Ruadh Mheallan 16, 67 Rubh Aird an Anail, Loch Maree 12, 23 sandstone dykes 65 scapolite 18, 24 Scourian 3, 6, 8-9, 11-19 Scourian gneisses 3, 4-6, 11-16, 67 Scourie 6, 67 Scourie dyke swarm 3, 7, 16, 21-27 dating 5, 7, 9, 14, 67-68 and Laxfordian intrusives 45, 47 nomenclature 8-9 shear zones in 51-52 strain markers 21, 25 structural chronology 14-16, 51, 61 Scouriemore 5 semipelites 7, 29-32, 44, 68 serpentinized olivine 12 shear zones 8, 51, 58-61, 62, 67, 70 Ard lalltaig 17, 18, 58 Flowerdale valley 52 Inverian 17, 18 Langavat, South Harris 8 in Scourie dykes 51-52 Shieldaig 17, 21, 60 Shieldaig dykes 26-27 Shieldaig Farm 65 Shieldaig gneiss 13, 29, 56 Shieldaig Lodge 12, 21, 33, 45, 48 Shieldaig marble belt 33, 48, 60 Sidhean Mor dykes 23, 24, 25 Sidhean Mor gneiss 13 Sidhean Mor quartz-chlorite schists 36 Sidhean Mor sulphide deposit 43 silica 34, 35, 36, 44 siliceous schist 52, 54 silver 43 skarn rocks 34 South Harris 4, 8, 51 Southern Region 3-4, 6, 7-9, 16, 67 spessartite 34 sphalerite 42, 43 sphene 12, 24 stilpnomelane 58, 60 Stoer Group 65

80

structural chronology 3-8, 14-16, 67-68, 70, see also Laxfordian subduction 27, 44, 68-69 sulphide deposits 35, 38, 42-43, 44 talc 43 Teangadh Buidhe Mhor 43 terminology see nomenclature terrane model 3 tholeiites 14, 26, 27, 40 titanite 13, 24, 36, 39, 45, 47 Tollie antiform 8, 12, 13, 14, 47-48 and Laxfordian 8, 47, 54-55, 56, 57-58, 61, 70 microgranite sheets in 47 Tollie dykes 21, 22-23, 25, 26, 54, 56, 57 Tollie Farm Fault 17, 65 Tollie gneisses 12, 13, 14, 17-18, 56 tonalitic gneisses 4, 5, 6, 7, 11-14, 67, 68, 71 Torngat orogen 1, 71 Torr an Easain crush belt 54, 58, 60 Torridon 7, 8, 16, 21, 52, 55, 67 Torridon antiform 8, 57 Torridon (Diabaig) shear zone 56

INDEX

Torridon Group 65 tourmaline 29, 31. 35. 36. 43 trace elements amphibolites 40 banded-iron-formation 35 carbonates (LMG) 34 garnet-biotite schist 36 quartz-chlorite schist 36 Scourie dykes 25-27 semipelites 31-32 'transitional' schists 46-47 tremolite 33. 40 trondhjemites 4, 6, 47 underplating 27, 67 volcaniclastic origin 38, 42, 44 zeolite facies assemblages 70 zinc 43